This isn't quite how it works.
PSTN (analogue) calls cross the intelligent network as single B-channels
(64Kbps). It makes no difference at all whose network it crosses, 64Kbps is
64Kbps, pretty much error free. (Incidentally, this is why V.34 is the end
of the road for analogue modems. The Shannon liomit means that the
underlying carrier must have at least twice the bandwidth of the signal
being carried, thus for PSTN that limit is half of 64Kbps which is 32Kbps.
Allowing for a few losses here and there, 28.8kbps is about the limit.)
The line that you are dialling in with is analogue only to the first
optimux/exchange. It is generally safe to assume that it will have the same
(analogue) characteristics from call to call, so if you are seeing different
characteristics for two different paths into the same modem, the problem is
almost certainly arising at the UTS end. Because 9514 is digitial all the
way into UTS's premises (I believe) the problem (if indeed it exists) is
actually with the equipment on UTS's premises. At a guess, this equipment
belongs to UTS and is therefore the domain of ITD.
Perhaps they bought bargain basement equipment.
"It often upsets a man's God fantasies to have (Misquoted? from )
someone shoot down one of his helicopters." (Ben Elton's "Stark" )