Vigor 2760Vn Firmware 3.7.5.1

I've got my Vigor plugged in to the BT phone line so that in the event of my SIP going down (eg Broadband failure) I can still make outgoing calls.

2 questions:
How do I stop incoming calls on the BT line line from ringing my phones connected to the router?
How do I set up a failover to the PSTN line if a SIP account is no longer registering?