You must ensure the ISDN2 is configured PMP, not PP. See the datasheet for the DX800A in the Gigaset WiKi which explaines this. Check with your BT Reseller on how the ISDN2 circuit is configured.
Get help with BT VoIP services working with Gigaset VoIP products http://www.WiKi.GigasetPRO.co.uk
