Please let me elaborate a little further because I can now understand that I missed critical info.
PBX -> Router 1 -> Router 2 -> SP
I need to route my SIP registration to my provider through a certain site (Router 2), so that is why I have tried to catch the traffic with different port in SIP register string.
The outcome is that Router 1 catches the packet and route it to Router 2, where Router 2 changes the port to 5060 before it sends it to SP (you can see the below screenshot).
So my SP validates my registration, because for them the ports are the appropriate.
I guess that the conflict is my PBX gets traffic in 5060, because my SP sends there my incoming calls and my PBX is configured in 50600. I thought that ports are used only for the packet routing so based on my NATing all would be forwarded properly, but from this experience it seems that the packet data section maybe has values based on ports (please correct if I got this wrong).
As business analyses, the request is to find a way to capture the packet so I can route it, so if anyone knows a value I could use from my PBX instead of port and capture it with my router (MikroTik rOS), that will do.
Maybe this is more of a routing subject but I am willing to understand if there is a way to distinguish SIP registration to the same SP from the same interface, not only based on authentication credentials.
I hope this message make things more clear than confusing.
As for the debug log, I have captured a section from the console but I do not know if this helps.
Thank you all for your time spending on this.