Cancel message while in 183 progress

Dear Guys,
I got on one of our pbx’s a very strange behavior. while the call is going out to our switch, we are giving back 183 and just after a sec the asterisk send cancel message to the switch and 607 message to the asterisk user.
I am almost sure it has something to do with RTP. on the same pbx same switch same destination I am getting> 0x7f87703e77d0 – Strict RTP learning after remote address set to: 1.1.1.1:12016
> 0x7f877042f430 – Strict RTP switching to RTP target address 2.2.2.2:8830 as source
> 0x7f87703e77d0 – Strict RTP switching to RTP target address 1.1.1.1:12016 as source
but on the faulty asterisk, I am getting only the first 2 lines and not the third one.
No idea how to solve it or even where to begin.
I also tried to remove the firewall and same result.

Hi,
I have disabled G729 and looks like it is working. no idea why when selecting g729 causes the PBX to pbx to cancel the call.