problems with an external modem

I have an external modem that will not work in one location but will in another. It’s a new usrobotics 56k (usually very reliable). The weird thing is that a cheaper LT winmodem works at the location where the external modem will not. It will not detect dial-tone. I tried tone and pulse, turning off compression, hardware flow control, different cords. This same modem will work at my office on the same PC that it will not work on my clients location. Any help?