Voice Over MPLS audio issues

A few weeks ago we stablished a 4MB MPLS with our VOIP provider. 50% bandwith was assigned for voice in the QoS profile. We only use this link for voice, no data at all.

When calls are routed throught internet every goes fine, quality is aceptable but when we switch to the MPLS connection we experience choppy audio for caller and callee. When MPLS is plugged, what is weird is that when we use silent monitor for listenig into calls, we hear that local calls are choppy eventhought this is supposed to be inside the LAN.

Our MPLS provider has done all necesary tests to determine that there are no lost packets between the edge routers. We did the same for our LAN. We use the same LAN for both links. this is not a bandwith issues because we are running under 40% of the bandwith capacity.

We did a test with an Audio Codes SIP Media Gateway (we used g711) connected directly to the MPLS Router and sound quality is fine, no choppiness. To discard that our LAN was causing the problem, we did the same test connecting the Asterisk straight to the MPLS router and a Snom IP Phone directly to the Asterisk box and audio was choppy (using both g711 and g729).

So my gues is that the problem is in the box.

Box Specs
Freepbx 2.5 - Asterisk 1.4
Xeon Dual Core - 4 GB RAM 120 GB HD

I appreciate any help.