I'm replying myself with the fix for my case in case anyone is interested. Basically you have to remove both the ras and the h323 helpers
This showed me the pointer:
id=20085 trace_id=1730 func=__ip_session_run_tuple line=3241 msg="run helper-ras(dir=original)"
config system session-helper
delete 2 #h323
delete 3 #ras
After deleting those helpers, the phone could register without more issues