The 5060 is already set as a virtual server. The problem is that the range 10000-20000 (for RTP) can only be port-forwarded (and not virtual server-ed) being a RANGE. That's the only difference between virtual servers and port forwards on this router. It's probably what's not working.
Looks like the Trixbox (also using a Trixbox) connects to the external VOIP provider, calls initiate but don't actually go through.
What hardware/firmware do you have?
I'm using a basic Intel Atom desktop POS for my PBX... don't need a lot for a home phone.
I'm using Linksys SPA-941 telephones
I'm using Server 2003 for DHCP and DNS
I use CallCentric for my VoIP provider
I DO NOT NEED TO USE RTP servers
the only port I have open for sip is 5060.