One-way audio for inbound DID call in double NAT setup

dear sir,

i have always got one-way audio issue when calling a local inbound DID number and forwarded to one of the internal IP phones.

my network is a double NAT environment, Thomson TG585 V7 modem (with NAT and as gateway) <-> Linksys WRT54GL router (with NAT and as gateway) <-> IP PBX server (virtual pc), IP phones.

The version of telephony systems are Asterisk 1.8+, FreePBX 2.9+ and A2Billing 1.9+.

If remove the Linksys NAT router, connecting the IP PBX server and the IP phones to the Thomson modem directly, then can get two-way audio when calling that inbound DID number.

It is very likely that the Asterisk 1.8+ can’t handle this inbound DID call properly with this double NAT setup.

It is much appreciated if anyone can give me some feedback and comment on this.

Thank you !

I suggest to activate SipDebug and post the trace of the Sip messages.