So if the packets are marked DF (Do not fragment) have you considered changing the source of the packets MTU size? IE Callmanager or CUCM Expressway gateway.
You can changed this in the router configuration as well. Note changing a CUCM MTU size will cause the network interface to restart.
I have not look at your pcap but what is your full topology, and why can your provider not accept default MTUs? They might be able to change on there side... if you are using default MTUs should be something they support if the traffic is making it to them and they are truely the ones dropping the packets.
