Change trunk registration port 5060 to 5061

I would like to use three trunks at three providers. Since my router does have some problems with all traffic on port 5060 I would like to change the registration ports to 5061-5062.
I tried several things but somehow the registration keeps on using port 5060.

Bellow you find changes I tried:

register string:

username:[email protected]:5061/username

When I check the log:

Via: SIP/2.0/UDP my.pbx.ip.add:5060;branch=z9hG4bK2b24e501;rport

How can I force to register via port 5061?