Any BT bods on here ??

Just swapped over landline and Broadband to BT from Sky..... cheaper deal etc

but have only just noticed when I receive a landline call phone continuously rings.... not the usual "ring ring"........ "ring ring" pulsed tone......
now its just a "ringggggggggggggggggggggggggggg" !
apologies for description but hopefully you get my drift ?

I've done the removal of front plate and plugged into master outlet test.... phone dial tone and calls all ok.... any idea ?? I'm assuming master knackered (capacitor) or circuitry within socket....
No overheads feeding house all fairly modern (15yrs) lines etc ? BT say lines all test ok ?
I'm a electrician by trade and can test any internal wiring (in this case) just a small extension of about 3 meters to second socket.