Are the helpdesk people only using phones or are they IPCC agents as well?
Basically, each G729 call will consume about 30 kbps over MPLS, so assuming that all users are on a calls you will need 300 kbps. If you don't anticiapte a lot of growth for VoIP traffic at that site you should contract for the lowest Real Time CoS possible, i.e. 10%.
HTH,
Chris