Sometimes, on a noisy phone line or even on a line with call waiting enabled, your modem may hang up because it thinks (incorrectly) that it lost carrier.
There is a setting on most modems for determining how tolerant it should be to temporary losses of carrier. On a USR Sportster for example, this is measured by the S10 register in tenths of a second. To make your modem more forgiving, you could add the following send-expect sequence to your dial string:
set dial "...... ATS10=10 OK ......"
Refer to your modem manual for details.