Hil
It could be a couple of things.
The master socket needs replacing or the voltage is to low on the line.
Either way an engineer is needed.
If you can ensure phones are not faulty.
All extentions are removed from master so phone plugs directly into master socket.
You can report it on 150 from phone if still ringing out.
You can also check it has correct number on it by dialing 174 and number will be repeated to you.
John
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