Depending on your exact set up, you may be able to achieve what you want. There is not really enough information in your post to really answer your question. But, I'll take a go anyway!
Key is likely to be how is the remote VoIP destination contacted ? If you're over the internet and you want some way to keep contact despite the WAN IP and route changing, you're unlikely to have much success at keeping a live call going - exactly as already mentioned.
However if you are connected to the destination via a VPN, then it is possible with something like the Draytek and the latest firmware updates. The VPN configuration has now added GRE tunnels to bond 2 VPN links. You could create a VPN dedicated on each WAN interface (i.e. forced to use each specific WAN link only) and then create a GRE tunnel to bond them into a fail-over link. This keeps the VPN running, even in the event of a WAN failure.
I'm not sure it would be readily achievable in any other way - even the dead link detection time would probably kill a VoIP call due to packet loss. This solution is also limited to when your VoIP destination is on a VPN - something you would not readily have available for a commercial SIP provider.
