I have a stateful ser with mediaproxy at public IP, client A(SIP B2BUA) with public address and client B(user phone) with private address.
1. SIP dialog already established and client A send re-invite without SDP (reinvite is necessary, problem was described in rfc3725.txt).
2. client B respond OK(200) with NATed SDP
3. Client A send ACK with normal SDP.

At step 2 in ser we can determine that we need to use media proxy and we start it.
But when we processing ACK, we see normal SDP, and won't to start mediaproxy. Thus we have one-way audio.
I tried to use flags, but it seems that when ACK arrives all flags zeroed. (Ser imply that ACK don't belong to invite transaction?)
I can't use AVP because commands are unavailable in onreply_route. The only way I found is to use global flags.
But this way is worse because foreign ACK can be processed between our OK and ACK from the same transaction...