Broadband OK, but phone won't ring

Apologies if there's a better place for this, but the heading says it all really.

Last day or so the phones here having been giving a half strangled attempt at ringing then going quiet. Voice connects OK ( if you pick up before the caller hangs up) and internet connection is OK.

We've hooked another phone on the first junction box in the house and the same happens there - just a brief flash of the light on the phone but now no ringing. This isolated all internal wiring so I don't think it's an internal fault.

The BT checker says it doesn't see a fault on the line.

Is there anything else I can check or am I doomed to have a BT visit?

Known as ring trip and due to low insulation