Yes, thats using Voice Over WIFI (an optional part of LTE). You can do this with an outbound policy but you need to be applying the policy to the VPN not SIP traffic as you cant see that (its encapsulated).
If you look at active sessions when you have a call in progress, you will probably see a session on port 500 or 4500 from your device outbound. That is the traffic you need to apply policy to. See here: