Sometimes that clears up the problem. Also make sure that your modem is compatible with the ISP's - some modems work better together than others. I always ask the ISP for a modem recommendation before buying. 3COM has a line test phone number that purports to tell you if your line is good enough for 56K transmission. It requires a one minute long distance phone call.
Some modems can test themselves.
The lower the number the better the connection. Anything over 40 or 50 means you're about to be disconnected. Not all modems support this command. Another trick is to tell the modem to be less picky about line noise. This involves modifying the initialization string sent to the modem before dialing.