Phones will continue to ring when some modem devices (like an ATA) are connected into your telephone circuitry if there is a problem with your internal wiring.

This is caused by the bell wire shorting with a leg of the line - most commonly caused by the cable being pierced by carpet grippers.

The way to test this is to undo the 2 screws of the master socket faceplate, pull the faceplate forward and then plug the adaptor into the plug behind the faceplate.

(The ATA must be connected into the socket using an ADSL micofilter of course and it is worth replacing the filter first as a faulty filter can also cause this problem).

If your adapter now works, then it’s definitely a problem with your internal wiring.