This is quite likely to be related to the CO, your local loop, or trunk
lines. Generally, if one customer is having problems, their modem,
inside wire, or local loop is defective, but don't bother telling them
that :-)
If several customers from the same general area, who have different types
of modems, are having problems, they may all have marginal local loop
wiring, or there may be bad trunk lines between their CO and yours, or
multiple AD->DA conversions (mix of digital and analog trunk/switching),
all pretty much impossible to get the phone company to own up to, much
less fix.
Sometimes higher quality modems (aka USR Courier) can help in this case.
If just about everybody is having problems, but the same modems work
fine elsewhere, your CO, local loop, or inside wire may be defective-
or perhaps your POP is next door to an AM radio station, or built on
a sacred Indian burial ground...