does anyone know the likely cause of problem I am having with Beocom 2 (mk1) ?

From time to time, when answering a call, the phone wil turn itself off. It also periodically shows messages on the screen: a) no contact with base b) battery needs recharging.

I assume (dangerous thing, assumption!) there to be an issue with the battery? Does anyone know how to remove the battery, where to get a replacement and what kind've cost?