ModemTest

From PassMark Software:

PassMark ModemTestâ„¢ is an easy-to-use Windows based application that allows users to test their dial up modem. ModemTest performs low level testing that is independent of your Internet Service Provider (ISP) and the TCP/IP settings on the computer. It allows the modem to be tested in addition to the phone line the modem is connected to. ModemTest provides a method for system integrators and computer repair centers to test both internal and external modems. By a process of elimination home users can also use ModemTest to help isolate Internet connection problems to a faulty modem, or phone line or their ISP service. ModemTest logs any data corruption and incorporates a modem speed test which reports the initial connection speed and the actual throughput.