There are some system checks you can use to see how good your ISP and Phone lines are.
You either net OEM SR2 or the ISDN Accelerator Pack 1.1 installed

Make sure SYSMON is installed. You can add this through the Control Panel / Add New Programs
Run SYSMON
Select Edit / Add Item
Click on Dial-Up Networking Adapter
Select CRC Errors
Click on OK
If you have lots of CRC errors, either the phone lines or your modem is having problems.

You can also add Framing Errors and Timeout Errors if you are still having problems and your phone line appears ok