I have an issue where my OCS installation is performing very poorly most users, we currently host all of our servers in a datacentre with plenty of bandwith, we have one site that has a site to site VPN to this data centre, users from here can call all over users who are using the standard VPN client to connect to the data centre, however if your connected via the VPN client and calling someone else who is also connected via a VPN client calls have very high stutter and are unusable.
Per your description, it looks like caused by the bandwidth.
What about the users in your datacenter site? Does the same problem happen to them? If not so, in my opinion, although your servers in datacenter with plenty of bandwidth, the two clients who built the connection were all in the branch site. So what about the VPN bandwidth between the branches site and the datacenter? Please check out whether It is stable or not. Does any packets is lost?