After investigating a bit more with the masquerading problem, it seems that the system does fully weird things.
The external IPs are sometimes reversed, sometimes not, and sometimes some traffic is reversed but other is not reversed.
For example, IAX traffic had the bad external IP, but SIP traffic had the good one, or the opposite.
I tried to stop and reactivate the forwading in the GUI interface, and this inverted the external IPs !!
I think that a bit cleaning is necessary here. The multiwan system is fully buggy.
I’m not sure this is a zeroshell problem. Perhaps simply a linux problem with masquerading multiple interfaces.
At first, i would allow to use SNAT with a static address for each wan interface, this should most problems.