A busy life has meant I have not been able to progress this issue as fast as I would have liked but it is critical that a fix is found quickly. I am a step closer this evening because I have achieved success, of a kind.
There was one test I should have done but omitted. I previously reported that with ONLY my old cable telephone plugged in to the Master Socket, my landline worked OK. This evening I plugged in ONLY my BT 3550 network (base station + 3 wireless handsets) and it worked OK. With BOTH cabled 'phone and '3550 plugged in, my system goes dead, yet they have coexisted happily together for the last 7 months with no changes to my system. Broadband has always worked.
This afternoon I received a call from TalkTalk Customer Care and I gave them a full progress report. The agent said he would leave this case 'open'. I feel there is still a mystery but at least I can now be reached again by landline. I have abandoned the cable telephone for now.
I thought members might like to see my home telephone network and I have attached a JPEG schematic showing its configuration. The extension booster used to work with 3 cabled telephones upstairs but I now have just 2 wireless phones upstairs.
I chose to raise the issue with the BTCare Community Forum because all my equiment is BT and the problem is of a technical nature. I encounter Openreach engineers moreorless every day but I have yet to meet a TalkTalk engineer, if they exist. I doubt very much they would have the knowledge and experience of an Openreach engineer. Being ex-BT myself I know who I trust.
If a solution was found it is highly likely that another BT customer, with a siilar problem, would stumble upon this thread and hopefully help them solve their particular problem.
I have given you credit for suggesting the swapping of the corded phone with the base station. I had it in mind to do that anyway but I sensed you and I were thinking alike and I'm always willing to try out a helpful suggestion.
I regard this problem as solved and hope that another BTCare member will provide the answer to the question uppermost in my mind. I still believe there maybe an exchange or cable fault but I will take that up with TalkTalk when they next call. If I can convince them I'm correct it will be Openreach that takes the credit.