Binding call control and/or media to a physical interface is a problem given your requirement. If the interface goes down then the address your signaling and media channels are bound to is not available. You need create a loopback interface (logical interface) on voice gateway and bind to the loopback interface. You can also have more than one loopback interface.
Now, in your case I am not sure which failure points you are referring to when you say you want to keep the calls up and active. On the ISDN side, if the E1 that is carrying the voice channel for an active call is disconnected that call is dead. On the IP side, it really depends on what your carrier is providing. Meaning, you can eliminate single points of failure in your network but you should look at what happens if the CPE fails or the connection from the CPE to the provider. Can you get to the same provider's SIP proxy servers in the event of an issue with a connection to the carrier. IOW, if you lose ISP-1 will ISP-2 automatically participate in a dynamic re-route of UDP streams that constitute the media channel of an active call. If ISP-2 is able/willing to do this, is ISP-1? Maybe this is way outside the scope of your question but if not, then you will want to play out scenarios with your ISP(s).
HTH.
Regards,
Bill
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