SIP Trunks with NEC UX5000

We currently have 10 offices being served by a SIP trunk coming in to our Corporate Office and services being distributed over MPLS.  I have the packets tagged as real time on the PBX.  When we reach anywhere around 60% port utilization on the MPLS pipe, the call quality dramatically suffers.  

The UX5000 was qualified as an eligible piece of hardware by our MPLS provider, yet nobody can figure out what the problem is.  While I want to replace the system, I don't believe it is in the budget.  We are serving well over 250 handsets to the 10 offices I identified.  

Our network team is pointing the finger at the PBX and I point back at the network.  I imagine it's a combination of both.  I used the configuration guide provided by the vendor, yet no change in quality.  I have no audio compression enabled at this time.

I'm looking for any possibilities to improve the quality.  Our vendor has limited exposure to enterprise setup for the NEC system and has no additional input.  I can't find any other vendors that are knowledgable about the UX5000.

Any insight would help