2 possibilities
1) it is the network
2) it is the network
<kryten> I know that opt 2 i the same as opt 1 but it is such a biggie i though it was worth mentioning twice).<kryten>
on thing it could be is routing issue , if the Handsets do not have a direct route to the Sip Server then this could be your problem, it can be proved by disabling Direct media path on the handsets
NOTE this is smiply way to prove the issue is a routing issue, you still need to correct the fundamental problem or you will have issues with call capacity as you overload the IPO by forcing it to unnecessarily route the extra data
Do things on the cheap & it will cost you dear