I have a US Robotics (3 Com) modem. I have tried the init string but still get inconsistent results.

Some model US Robotics modems have difficulty communicating with the computer at low baud rates (i.e. under 9600 baud). Try setting the baud rate on the carrier setup screen to 19200. Also use the init string given to lock the baud rate between the modems to a lower rate.