I've got myself some new DECT phones (Siemens Gigaset S685IP) which can also connect to VOIP and have successfully set them up with my BBV voip lines. I can initiate outgoing and answer incoming calls to start with, but either way after about 45seconds the call cuts off and I get an error message on the phone reading "IP Status, Code 701". It also seems that it takes about 10s for sound to cut in on an incoming call (outgoing calls have sound from the outset).
Have you looked at the call stats on the router? That may give a clue.
Alternatively, try the VOIP helpdesk - 0800 1691146 whio should be there 24hr/day
See here for full configuration information when using Gigaset VoIP products with BT VoIP Service