Hello ohdude,
** The following is based on my knowledge of Asterisk - not on Zentyal VoIP module - Still, it should basically be the same **
It sounds like your firewall is blocking RTP (no pun intended).
Try allowing port range 10000 - 20000 on your firewall.
Explanations
Strictly speaking, this shouldn't be required, but different adapters and such use different ranges. Generally port 10000 is where they start (these are for RTP for the actual voice packets), and you usually don't need more than a handful, but providers often tell you to forward the whole lot to make sure it works.
i.e.
The defaults for the PAP2T are as follows:
SIP Ports - UDP Ports 5060-5061
RTP Ports - UDP Ports 16384-16482
The 10000-20000 number just "covers all the bases".
Please share your feedbacks and results with the community.
Best,
Marcus