QoS packet marking is provided on the IP packet header. When operating across a GRE tunnel, the packet marking needs to be copied to the GRE header so that the intervening public network can appropriate deal with the high priority voice and signalling traffic.
In addition to the usual considerations for QoS over VoIP, these additional considerations need to be made:
Ensure that the packet marking for the IP packets is being copied to the GRE tunnel header.
The recommended way of configuring this is to issue the qos pre-classify command to the tunnel interface, and add the service policy to the physical interface.
Ensure that the public carrier is capable of providing QoS based on the packet marking you provide.