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I have solved this now.

We had previously created custom udp/tcp ports for SIP to make things work with this specific VoIP provider. In these custom protocols we left the protocol undefined and just entered the port ranges. This was to avoid problems with inspection that we had experienced in the past. Suddenly this problem with the 2nd-phone-to-register-does-not-get-it's-payload-NATed-issue appeared anyway. Finding no way forward with the current protocols I decided to try to clone the default SIP UDP object and enter the port range there and then change the inspection settings according to below:

It was checkpoint support who gave me the idea. I had already told them that we don't inspect SIP because we don't use the standard protocol objects but when they suggeted this change despite the fact, I decided to change to the inspected object and try this and then rebooted the mgmt server and the appliance. So this was the solution.

Hope someone finds this before spending two days on the issue 😄