I am currently using a different router for a customer with 3 WAN connections, cable, DSL and a T1. With all services up, this is how it's set to operate:
-VoIP traffic flow over T1
-All other traffic flows over cable
-(DSL is stand-by only)
If the T1 were to go down:
-VoIP traffic flows over DSL
-All other traffic flows over cable
If the DSL and T1 are down:
-All traffic will flow over the cable internet
If the cable goes down while the other two are up:
-VoIP traffic flows over T1
-All other traffic flows over DSL
I was able to accomplish this behavior reliably by identifying the traffic by destination IP (VoIP provider) and setting up various probes through each WAN connection. This type of system is what I was referring to.