Different Request-URI in INVITE and ACK

Hello,

 

I'm trying to understand a strange behavior where the SBC is dropping the ACK which follows a 200OK in a call scenario.

 

I have captured the packets of this call and have found that the only strange think is that the Request-URI is not the same on the initial INVITE and the ACK that is being dropped.

 

INVITE  sip:+99aaa

to: +99bbb

from: +99ccc

 

ACK sip:+99bbb

to: +99bbb

from: +99ccc

 

According to the RFC the Request-URI should be the same as the TO but the invite is not being dropped, the only thing that can think of is that the SBC can't correlate the INVITE with the ACK even though bot come from the same UA, it thinks that the ACK belongs to an unknown call and drops it silently.