Many phone systems in North America use an audio format known as u-law. This nonlinear format supports a few different sample and data rates and is saved as a .wav file. (the .wav header can specify a number of different formats, u-law being one of them)
Without knowing the details of how your particular system works, give it an 8-bit 44.1 kHz file and see what happens. It may be able to work with that format or may do an internal conversion. If it doesn't work, you should try 8-bit at a sample rate of 8 kHz, which is one of the default formats specified for u-law.