Since they've replaced (in effect) the lines all the way from the CO
to your protector, one would have to assume that your poor signal is
due to either a) the house wiring - perhaps have another line run from
the protector to your jack, preferably via an alternate route, since
there is the possibility of ac induction and all that. b) the
protector itself -- when they swapped the lines, the might have left
the modem on the original protector ... or c) the CO equipment from the
MDF (frame) to the switch.
And yes, it's sad but true that the telco's don't believe that our
modems should operate satisfactorily on 'voice' lines above 2400bps. I
suppose you could tell them that you are operating at 2400 "baud" and
not 14400 "bps", since (I hope I'm not out to lunch here) high speed
modems do indeed operate signalling at 2400 baud. Phase/freq coding in
the 2400 signal gives us the high speeds we so dearly love (and we
refer to this as "bps", or the effective throughput).