OpenVPN OK - Sip client not OK

[nsa2015]

Hello,

I use pfsense 2.2.4-RELEASE just for OpenVPN in my DMZ.
I only use one network card (WAN).

The OpenVPN work fine and the client can connect and use all the network ressource.
But I have a problem when I try to use a SIP client trought this VPN Tunnel.
The SIP client can auth and receive call but not make a call.

There is no firewall rules (the client can ping/ssh the IPBX and with tcpdump, I can see that the SIP traffic is correct)
The NAT is not active.

Whith the same computeur, the same SIP client but another OpenVPN (juste Debian + OpenVPN), it's work without any problem…

Could you help me ?

Thanks