As tested with iptel and linphone ToIP's services, gnome-calls is unable to register :
account encountered an error : internal error occurred
tested on two different phones with postmarketos.
so I'd recommend to just use the linphone fork rather than the freeswitch fork.
(We should check that certs are validated correctly there but that's a separate issue)
This is likely not a freeswitch sofia-sip issue but just triggered by the fact that there's no attempt being made in calls to set up the certificates for validation.
As tested with iptel and linphone ToIP's services, gnome-calls is unable to register : account encountered an error : internal error occurred tested on two different phones with postmarketos.
as advised by guido, i add this issue to your bugtracker https://gitlab.gnome.org/GNOME/calls/-/issues/650#note_2167016