I have been using VoIP with Sipgate on my Vigor 2800VG successfully for years but a few weeks ago it started behaving oddly.

When a caller dials into my Sipgate assigned number, the hard-wired handset plugged into the router's port rings as expected. However, if I don't pick-up and the caller hangs-up first, the receiving handset carries on ringing and continues to ring alone for a few minutes.