Quick preface here: There is a TON of information around on modem performance. I once spent a good month messing with scripts to find the "ultimate" connect string for a BestData 56K speakerphone modem I own. This section will be expanded and added to (I haven't found all my material on backup yet!), and if you have some good info to add to it, please email it. Personally I'm convinced that the biggest problem plaguing connect speeds for users is old phone lines or use of Pair Gain. I hope you find this section useful and hope you get faster connect speeds as a result of something here...enjoy! :-)

Telcos, Pair Gain, Grounding and Modem Performance:

What you should know about Pair Gain:
Telephone companies routinely use Pair Gain devices to serve their customers. What is Pair Gain you ask? It is a method to serve more customers over the same number of cable pairs. Pair gain is a digital service. Ma Bell can cram up to 96 conversations onto 11 copper pairs as I recall, using Pair Gain. This helps keep the cost of service down which is good but if you are served by Pair Gain you will suffer.

Here's how this works. The copper pair leaves your home and is run a short distance to a small hut or metal cabinet that contains the Pair Gain electronics. Here your analog signal is converted to digital and multiplexed onto a digital facility (T1). At the telco Central Office the digital signal is converted back to analog and cross-connected into the telephone switch where it is converted back to digital once again. (Note that this is two A/D conversions.) You cannot make a V.90, X2 or FLEX connection in this case.
What this means is that you will be unable to achieve the bandwidth your modem was designed to deliver because the phone company facilities will not support it. Typical telco response: "We only guarantee 14.4 kbps data transfer speeds." Or, “We don’t guarantee data performance on voice grade lines. If you wish we will install a conditioned data line.” Good solution if you like spending lots of money. If you want better speed, and DSL or Cable modems are not available in your area your only alternative might be ISDN which is becoming more widely available (but still very expensive for the speed you get).

If your performance suddenly drops to 28.8k and stays there it is likely that your telco has just put your line on a Pair Gain system. All you can do is talk nice to them and plead for being put back on copper.

Other performance issues:

Poor performance can often be traced to poor connections either in your home or outside. Before you call your telco you should try removing the phone wires at the distribution point in your home. (Where your telephone cable enters your home.) Some of these have jacks to simplify testing. Next run a direct cable to your modem and do some testing. Does the speed improve with your internal home wiring and phones disconnected? If it does you can rewire and/or replace old or faulty sets. If not you should start bugging your telco. Dial a silent termination number (frequently XXX-0011) and listen for any trace of noise. Listen for clicks, static and especially hissing or hum. If you hear any of these report this to the telco and keep reporting it until they respond to the problem.
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Another issue that is often overlooked is proper grounding. I am not talking about the three prong AC outlets in your home here. I’m referring to your homes main electrical ground connection. This is often located in a damp basement and is usually a connection to the cold water pipe made where it enters the home. This connection is frequently corroded and can induce noise. If a visual inspection makes you suspect that this connection is a possible problem call your electric utility and ask if they will service this connection. If not you should hire an electrician to reconnect the ground.

When dealing with large regulated utilities it is often good to appeal to their sense of fair play and not go off on a tirade. You can, as a last resort, seek assistance from your public utility commission. Regulated utilities don’t like getting questions or complaints from these commissions. The commissions often will remind the utilities of the number of complaints they receive when the utilities ask for rate increases.

Modem Security Issue on Macs:

Go to this link on Macintouch and read about the modem guard/security defect in Mac modems that was discovered in June 1999...there are ways there to test it and to fix it if you have the problem.

More Tips:

Try Adding 3-5 commas after the dial-up number in your configuration. This increased my connect speed from a flat 36k or 44k to up to 50,667.

Make sure your QuickTime Control Panel setting is the speed that matches your modem (don't ask why, but some people reported this fixing connect and speed problems with modems...look on MacFixit's forum, under Internet troubleshooting, for more on this and other modem issues).

Go to 56k.com for lots of resource references.

Try this one too: Modem Initialization Select-O-String (and More).

Try the Apple Modem Script Generator. It’s a bit dated, but at the very least it has a lot of info about what the segments in a modem init string are for.

And finally, read the below...I can't take credit for it, and I can't remember who posted it.

Here is what apple support told someone on the iMac tech exchange board, and its good advice. Remember that connection speed means (almost) nothing, its your throughput that counts. The problem could be your phone lines, or your ISP as well.

Where modems of different makes, models, firmware/flash-ROM revisions, and chipsets are concerned, one modem may have an incompatible method of implementing a connection that it has negotiated with the other modem, or the negotiation itself may fail.

For example, if a modem cannot sustain the current connection speed but is unable to successfully negotiate a slower connection with the other modem, the connection will incur excessive transmission errors, decreasing throughput and potentially forcing a disconnect.
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A) Do a flash-ROM upgrade on your modem.
If your modem can be flash-ROM upgraded via software, apply the most recent modem update available. Apple 56K modem updates are available online. Check your modem manufacturer’s website to see if there is an update available. The latest firmware code may make the two modems more compatible—or less.

B) Disable 56K protocols in favor of V.34 if necessary.
Limiting the modem in this way may reduce or eliminate the need for the modem to make as many adjustments in response to poor line conditions, potentially sidestepping interoperability issues altogether. This change is incorporated into the “V.34 Only” modem scripts available for some Apple 56K modems. A variety of other modem manufacturers supply similar ARA modem scripts for their modems. If you are connecting to the internet using software that does not use modem scripts, supply the software with a V.34 initialization string appropriate for your modem. Your ISP may be able to assist you, or you can teach yourself the proper initialization string by studying the AT command guide provided with your modem or available online from the manufacturer.

C) Query your ISP about different dial-ins available.
Ask your ISP whether different modems are available via an alternate dial-in number, or switch to an ISP that provides different modems (call them first, as they may already know of issues they are having supporting the modem you are using). Perhaps borrow a friend’s Internet account, obtain a trial membership with another ISP, or connect to a PPP test server if you know of one. If all else fails, you may wish to go so far as to try a different modem yourself—one that your ISP recommends for best compatibility.

D) Reduce signal interference & distortion if possible.
The telephone network path between modems must be sufficiently free of noise and frequency distortion to permit a stable connection. The modem analyzes these line impairments as they affect the quality of the transmission signal. Poor signal quality causes transmission errors, reduced throughput, reduced connection speed, and dropped connections.
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E) Experiment with eliminating potential sources of line impairments in the home.
* Any device connected to any telephone outlet, especially if used by the modem, and even if on a different line: telephones, including cordless telephones and their base stations, answering machines, fax machines, caller ID boxes, and other modems.
* Telephone line splitters, cable extenders, faulty or overly long telephone cables (shorter is better).
* Surge protectors, including those that offer telephone line protection.
* Alarm systems, especially those that are connected to the telephone system.
* Computer equipment, including speakers.
* Fluorescent light fixtures and light dimmers.
* Satellite dish receivers.
* Appliances such as air conditioners, refrigerators, dryers, microwave ovens, and televisions.
* Any AC power source and related cabling.
* All other things electrical, including problems with the premises wiring: loose or corroded connections, too many splices or bridges, insulation deterioration and exposed wiring, non-twisted pair telephone wiring.

Start with a direct, unobstructed, modem-to-outlet connection using a short, high-quality telephone cable. Better still, test with a direct connection to the external telephone box located at the back or side walls of the residence. It provides one or more standard telephone jacks supplying the home telephone connections but bypassing most impairments inside.

F) Listen for noise on the phone line.
Because the telephone company is only obligated to provide voice-quality phone lines, it is unlikely to address the many and varied off-premises sources of line impairments that can affect the connection but don’t cause audible noise. Connect a telephone to the wall jack used by the modem, then pick up the line, dial the number 1 to stop the dial tone, and listen. Contact the telephone company if the line is not quiet (loud hiss, pops, static, voices), but first disconnect other telephone devices and check again.

G) Use a local dial-in number to your Internet Service Provider.
Long distance and 800 numbers aren’t typically a concern, but may cause the call to be routed through more facilities, indirect paths, and different carriers with different types of equipment and lines. Local calls may be auto-forwarded in the same manner, but most reputable ISP’s don’t engage in this practice.

If you're a bit daunted by these varied and uncertain possibilities, consider carrying the computer to a local Apple service provider for testing on analog lines of confirmed quality.

Most of the time the issue is software related or there is an external cause, including the telephone line or ISP.
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Review the following Tech Info Library article which includes links to other helpful articles to help you troubleshoot and solve this issue.
Modem: Troubleshooting References (TIL# 43079).

If you want a more detailed explanation read on.

There is a difference between modem connection speed and throughput.
Why is this important, you ask? I won’t bore you with technical stuff, but it is possible to connect at 33.6 Kbps but have a throughput of 57.6 Kbps. (This type of throughput is achieved when connecting via protocol *v.34 and MNP5 compression.) The moral is you can have a slower connection speed, let’s say 33.6 Kbps, which does not necessarily mean the throughput is also slower. A modem script will connect at the fastest but most reliable connection speed and use the highest compression possible.

V.90 is a completely different story. To achieve a 56K connection is really to download at 53 Kbps, the highest allowed by the FCC, and upload at 33.6 Kbps. The download is faster because the ISP can transmit the data in digital format but your modem cannot transmit at 56K on an analog telephone line. The telephone company has installed digital lines throughout your city, but not everywhere. If there is an analog connection between your ISP and your computer’s modem the best the modem will achieve is ~33.6-44 Kbps throughput. The moral is that 56K speed is not always possible. It’s like calculus, you can approach the limit.

Okay, so throughput is important.
Since the connection speed is not always a good indication of the type of connection you have, it’s best to measure the throughput to get an idea how fast or slow things are moving. Do a search at Google.com, type “Connect Speed Tests” (no quotes) in the search field. Several links to speed tests will be presented. It would help if you can connect a different computer at the same telephone line to get a balance. Run several tests over a period of days to get a feel for your telephone system, taking into account peak times and all.

If you contact your telephone company tell them if you hear static on the line. They can come out and test your lines. In the process of checking they’ll verify all the connections between your home or office and their equipment to be solid and secure. Although they may not find an issue, just tightening connections is all they need to do. You may notice a difference when they’re through.
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Modem Scripts
You can try different modem scripts. Use the different Apple modem scripts Apple supplies. Test the throughput for each to find the best one for your telephone line and ISP combination.

You can try non-Apple scripts too. Besides those listed above, do a search at versiontracker for Modem Script to turn up quite a few 3rd party scripts.