[bluzman]

Where I used to work there was a tower that did that to our phones and we fixed it this way. Find out what frequency the tower operates at and attenuate (cut) that frequency (and/or the harmonics of that frequency) with a filter. Radio Shack and other places sell them. Get flexible aluminum conduit run your co-ax through it and bond the conduit to earth ground. Our local Radio Shack had the filters for the local tower in stock. It's worth a try.