How can I make my BT Studio 1100 answerphone work with wireless broadband?

I have a BT Studio 1100 with answerphone and one extension phone which have been working with no problems until I recently switched to wireless broadband. Since then the phones work perfectly but the answerphone has such bad interference that I can't make out the message. The splitter box and router are all correctly set up. The answerphone is part of the 1100 base station but there is no problem with reception on the phone itself. Have you come across this problem before and how do I sort it out?