G.729 Transsport not allowed?

I have both soft and hard phones that support G.729 but when I try to create a session between them I get an error in the client that the call is not allowed.
The setup is two clients with an Asteisk between them ( SIP) with Trixbox. Due to firewalls and NAT I assume the RTP as well as the SIP is being routed via the PBX, The question is, is this not working because I do not have a G.729 license on the Asterisk even though both end points do support the protocol?
Is there a way to make this work ?

voip-info.org/wiki/view/Aste … +pass-thru