i havnt done polycom over mpls
but i would like to give u hints to consider it
the first think u need to know from the ISP what COS they put in as priorty and how many COS they provide u based on SLA and how match max traffic then they start to drop traffic
the other think is to know how the polycom mark the traffic and u need to calssifiy it and put it in proper calss based on the classes provided by the mpls ISP
good luck
and hope this helpful