>Probably you could investigate grounding at your PABX, or in the worst
> case, install RF filters along some phone lines and see what happens.

 Do you have the schema for the RF filter??

 > 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.

 Can you can explain some solution tha you have found?

 Ciao

 Leo