RF interference in a 56K modem
 

Rather odd things happening. An external modem situated in an outside office
refuses to handshake with the ISP's modem. This does not happen when the PC
is brought inside and connected. The office phone faintly receives a radio
station (91 FM) but only when the modem is connected to the phone socket.
The phone doesn't do this on any other phone jack. The internal modem that
this external is replacing worked fine, but had to be unplugged after use
else people could not phone in (connection would 'hang up')

I'm rather stumped as are Telecom and my ISP. Any help?