I am having similar problems and will seek an answer and let you know, if I can.
The BT Freestyle phones were excellent uncomplicated phones that were easy to live with. Unfortunately one extension phone lost its side-tone, which created a problem for us.
I recently purchased later model BT cordless phones... but they were not easy to use and I immediately sent them back. They were over complicated and not at all easy to use.
I have now succeeded in buying three new Freestyle 'additional handsets' and chargers and am having difficulty getting them to work with my existing, working, base unit. I will check through BT to see if they have anyone who can check them out for me. I suspect they won't want to know !
Tony Kent., February 2010
