First thing to do is listen to that radio and find out their prefix,
their physical location, phone numbers, talk to those guys, ask about
where are installed their antennas and so on.

It is not difficult to receive radio over simple wires and some silicon
components involved, mostly when the transmitting antenna is close by.
In 1988 I was living in S‹o Paulo City in Brazil, a crowded city with
sky crapes and all, 2 blocks from the highest hill, highest buildings,
highest TV, Radio and everything antennas. With my home stereo powered
off, I could listen clearly one FM station at the speakers. It
disappears when turning on the stereo, volume off, just silence. Stereo
off, radio came back loud and clear (well, not that loud, I could hear
the family room speakers from my bedroom at night)... Looks like the
output power transistors, along with the speakers and more than 30
meters wiring around the apartment, formed a detection system, resulting
in loud reception. The interesting point, is that it had more than 30
big antennas all around, dozen FM antennas, TV, microwave, communication
and hell, then why just that specific radio was entering my speakers?

>From the time I was working with radio transmission, I noticed that when
the transmission system is out of adjustment, say the output power stage
out of tune, antenna cable out of length, or antenna elements with
problems, you would have signal reflections all over the transmission
hardware. This creates heat, noise and interference.  As a radio
technician I traveled a lot fixing, adjusting and installing radios for
this company. Sometimes to tune the final power stage, I got without the
right RF power meter and had no other way than using a small AM
transistor radio receiver, tuned in the middle of two Am stations as to
receive interference. The best transmitter adjustments caused very
little interference to the AM transistor radio.

Probably you could investigate grounding at your PABX, or in the worst
case, install RF filters along some phone lines and see what happens. 
Using a scope and a differential transformer to locate where the radio
signal comes stronger can also locate your source of problem.  You will
find out that the solution sometimes is ridiculous.