It sounds like noise on your line. Pick up the receiver and dial a couple of numers slowly and listen of any kind of humming, static etc. Your line should be dead quiet and have a high signal to noise ratio (loud volume, basically) for a 56K modem to negotiate 56K - most of the time it would be around 50.2-52K.
Remember, bad weather that includes lightning specifically, causes clipping and scratching on the line that forces the modem to re-negotiate during connection.
If a 'clip' or 'scratch' occurs during the re-negotiation process, the process will terminate and the modem will hang up.