How can I fix my BT Synergy 3505 phone?

I have only had this phone for about a year. I have one additional handset. The phone was fine up until I went away on holiday and we switched if off for 10 days. When we switched it on again it went to the engaged tone while we were on the phone both for outgoing and incoming calls. We have tried switching it on and off again but this made no difference. The additional handset upstairs doesn't work very well either now.

We have lived with this for a couple of months now but instead of the engaged tone when we walk into some rooms particularly the kitchen the phone just cuts out.

Any help would be appreciated.