If the problem relates to voice breaking up, it relates only to the media path of the call, not the signaling.
You should focus your efforts on the RTP path and not on the protocol used on the ICT trunk.
You can get the statistics for the Remote IP, port and packet loss from the phone webpage. As well a packet capture from the back of the Phone with Wireshark will reveal a lot of statistics relating to Voice Quality
Then you should try to identify the traffic flow across your network, using show ip cef exact-route. or other routing verification.
Make sure your Switches and Routers are respecting the packet markings, and you can see the traffic been matched by your policy maps. Check links for congestions, dropped packets, buffers full.
About MPLS I would recommend to check the DSCP to EXP bits mapping, to make sure traffic gets treated across your provider.
If you can confirm a delivery of steady RTP with 50 packets per second, and little jitter and good QoS marking to the PE edge, and the other CE router you get higher than normal jitter and packet loss, you can blame the provider for not meeting their SLA.
As well you might consider using the new traffic export feature on the ISR to get a packet capture on the IOS itself, for your analysis.
It's mostly network and QoS troubleshoot what you are looking at.
Hope this helps.
