Dialling pulses on extension phone

[paul243]

I have Gigaset c620a cordless base with 3 handsets. All work fine.
However I also have 2 corded phones plugged into sockets, in bedroom and garage.
When answering an incoming call on the garage phone, after several seconds, there is a loud tone like someone is trying to dial on the same line, reminiscent of an old "party" line years ago.
This call also be heard by the caller. Seems to be only on incoming calls.
Any thoughts?
Thanks

 

[JohnD]

is the gigaset trying to cut in the answerphone?

 

[paul243]

Thanks for quick reply!
No it occurs when the AP is off

 

[sparkymarka]

Alarm system on the same line ?

 

[SpecialK]

Do you have broadband on the line? Is it correctly filtered? Every outlet needs a filter unless one is fitted at the master socket.

 

[paul243]

No alarm, broadband, tv and phone from VM . The incoming telephone sources from external VM box to original BT socket in an upstairs room and then around the house.

 

[Tally101]

Faulty line in contact with another line, have it tested