Actually, I've got to go with the Apple side of things here. Since upgrading to 10.2.5, my dial-up access has improved greatly.
Previously, I was running 10.2.3 on my iMac (17", 1ghz). The only way to establish and keep a decent connection (> ~42kbps) was to disable compression in the modem settings. Of course, that had its drawbacks, such as slooooww data transfer, and iChat no longer connecting. Upgrading to 10.2.4 only made things worse, with connections that could be established using normal settings only lasting ~2 minutes. Other common fixes, such as disabling echo packets or cranking the modem back to a v90 or v34 protocol did not help.
I know it was not my physical phone line, as I never had any problems establishing or maintaining a connection greater than ~42kpbs using the modem in my Ti-Book (867mhz), or even my old PII-400 W2K box.
Since stepping up to 10.2.5, the iMac consistently gets good connections that last however long I want them to using default settings.
Check out the Apple discussion forums for suggestions that may fix your issue. One of the more interesting topics that has been brought up by some is that some modems Apple uses (mainly in the newer iMacs) are overly sensitive to line noise. Some people have reported success in "fixing" their connection problems when all else has failed by placing an RF filter on the phone line. These would be the type of filter that is used with some DSL installations, and is available for about $15 at an electronics store or (shudder) Radio-Shack.
Good luck getting things resolved.