I guess its just giving a single ring that you cannot hear, but is sufficient for the phone to register as a missed call. You will just have to report it as a fault, and say that is still there with everything unplugged.
Hi, I can't see in any of your posts that you have tried the phone at the BT test point, this is located by removing the bottom half of the main socket, the test point is inside on the right hand side. I suggest you try this before BT visit your premises.
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