As a telephone technician and a broadcast engineer maybe I can give a few suggestions.
You are probably very close to the stations transmitter site, most likely an A M station. The telephone wireing in the building is acting like a big antenna.
Sometimes chokes in line at the phone system will help but I have had better luck using bypass capacitors on all lines going into the system. Normally I use .01 UF disk ceramic capacitors. Be sure they are rated to handle at least 100 volts. You can get these at most electronic supply stores. you will need 2 for each extension. Connect one side of the cap to each line and the other side of the cap to a GOOD ground. Keep all leads as short as you can. I have often soldered one of the capacitor leads to a bridgeing clip and just push it on the 66 block.This seems to be the easiest way.
If your phones are wired with the old untwisted pair cable then rewireing with a good twisted pair cable or shielded cable may help.
Be sure all of your grounds are tied together near the system. Power ground and telco ground should be connected together. Also have you tried another make of phone? Some of the cheaper phones are good phones but just can't be used near a radio transmitter.
Getting the RF or radio interference out of a system can be a nightmare. There is no sure fire way to do it. Sometimes the suggested cures I listed above can make it worse. You just have to try and see what happens.
You may try to talk to the engineer at the radio station. He may be able to give you some suggestions. Don't try to blame it on the station. There is a very small chance that they have anything wrong that is causing this problem.
I hope something in this long post helps.
Good Luck
Robert