Hi,
You should be able to get some improvements with their current equipment.
First thing to check though, is to make sure that your customer has subscribed to the correct type and amount of bandwidth on the MPLS link. You then need to know how the service provider classifies the different type of traffic, this will allow you to work out how to classify and mark the traffic on your 2801 and 1841.
If you dont know what the PBX is sending, then the easiest way to work it out is to get a packet sniffer to check out what is going in and out of the PBX network interface. I suggest using Ethereal, its free.
You should be able to use a combination of the source IP address of the PBX, and the TCP / UDP port numbers to help you classify and re-mark the traffic if required.
Cheers,
Tim