Sorry don't know obvious either fault or nearby interference.
The book does say keep the unit/s around 1metre from radios etc.
contact BT help on 0845 602 1041
If you haven't got the user guide then you can down load one from www.bt.co.uk/userguides then in the A-Z pick your telephone press the go at the end of the box and wait for it to down load. If you haven't got Acrobat reader it should ask you if you wish to download Acrobat reader [it's free] and of course it's needed to down load the user guide.
BOB, February 2005