So something is blocking the RTP... what is your configuration for the network? Is the MiCollab sitting in the DMZ or in server-gateway mode or
Also, in the TW blade there is an application called Teleworker Network Analyzer... download and install it on a PC and run it from on-net and then again off-net and compare the ports... it should give you a pretty good understanding of what isn't working.
And assuming this is in the DMZ it is almost always a firewall issue... Either missing port forwarding or some kind of IDS or similar feature blocking traffic. working with your firewall/router vendor and examining it's configuration and logs could be useful too.