Some possibilities:

1. Line noise. Pick up the receiver and listen for cackling or scratching noise.

2. Someone picks up an extension.

3. The modem is flaky. Try another if have it.

4. Bad phone cord or connectors. Move to another phone jack.

5. Bad modem on the other end. Try a different destination to see if the problem goes away.

6. Bad modem driver. Look for an update.

What rate are you connecting at?

Peter