Can you post your changes somewhere?
Yes, I've uploaded them to gerrit: https://gerrit.osmocom.org/#/c/osmo-mgw/+/12390/ Jenkins build failed as it needs to be compiled against libosmocore of branch laforge/iu_up. I've just uploaded it to make it easier to share the changes.
Even more so, can you post pcap trsaces of a working setup (whether or not osmocom)?
I cannot post the pcap trace of a call where commercial NITB was used. Though I attach a trace of a call of Osmocom system with the patch referenced above. Description: 10.0.3.20: nano3g ip.access S8 10.0.3.19: Osmocom System (where the trace was captured) 172.48.1.3: Kamailio PBX and rtpengine