Direct VOIP between two extensions (as opposed to via a proxy)

I have three SIP ATA's (PAP2-NA, SPA-1001, SPA-3102) behind 3 different linux firewalls on 3 different internet providers. They register fine, however it would appear that calls between them are always going through a pbxes proxy (instead of going direct). Before I start searching for which aspect of my settings and NAT / port forwarding is misconfigured I thought maybe I'd ask here for some pointers (side note: this worked with another provider before I switched to pbxes)... Are such direct connections even supported by pbxes?

RE: Direct VOIP between two extensions (as opposed to via a proxy)

Ahhh, welcome to the holy grail of NAT traversal with SIP Re-Invites, which are definitely supported by PBXes.

• Inquire whether your Linux firewalls support a SIP ALG (Application Layer Gateway), provided the WAN side of your firewalls is on a Public IP without NAT.
• If they don't, consider placing your ATAs in the DMZ of each router, provided the WAN side of your firewalls is on a Public IP without NAT.
• If the above option is not a valid course of action, port forward the UDP ports 16384 through 16482, towards the Private IPs of your ATAs.

In the first two cases, disable the STUN service, since it will conflict with the SIP ALG, while it should not be needed in the DMZ case.