Are the two places where it works/doesn't work very far apart? It occured to me, it might be due to phoning into different modem pools, where one lot might be rockwell or something and the other lot might be lucent or something and have different ideas about the way they implement V90, V92, X2 or whatever. I had heard something about USR X2 modems not liking to talk to other X2 modems. So it could possibly be solved or alleviated, by trying to force a different connect protocol, if for example you're using V92, try V90 or X2.
Just an idea. Could also be issues with phone line quality between the two locations.
Road Warrior