To cater for future power outages (we live in the sticks) I have bought an ex British Telecom landline phone, model Decor 100 dating back to around 2000.

It was advertised as working, and is clean and in good condition. Plugged in to my ADSL filter and hey presto....dial tone, able to make calls, perfect. However, the bell won't ring on incoming calls, although when I know there is an incoming call I can lift the handset and connect to the call. I am reasonably handy with tools, use of multimeter, etc, and the phone is easy to get into.....can anyone please suggest what the problem could be, and what i should do to resolve.