1.Use policy routing on the incoming interface for the voice traffic to catch and set the next-hop parametrs to the next-hop
of the chosen wan interface. This is not so cool as it add and additional processing delay to
voice traffic.
2. A better solution is to make the session target of the voip dial-peer equal to the next hop address of the wan interface. This you can use only if the next-hop
router at the end of the proposed wan link is the terminating router
for the voice call.
3. If not then use static router to point you traffic out the
next-hop of the favoured wan interface. add the second static with a higher administartive distance for backup or not if you n
have a dynamic route entry that can kick in.
ip route x.x.x.x 255.255.255.255 y.y.y.y
ip route x.x.x.x 255.255.255.255 z.z.z.z 200