The phone line quality is poor. Remove any splitters, shorten the cable and try again.
If the user is dialling out through a PBX system/switchboard test the line using a direct line if possible.
Make sure there are no appliances near the phone line which could cause interference and that the line is not coiled.
If none of these work try some strings such as:
S10=50
S10=100
S10=100S25=200
AT&F1X0S10=100S25=200S38=2