Sipgate rejects "sips:" Contact, "sip:" with transport=tls works

[airtower-luna]

I'm trying to get Gnome Calls (its SIP support is based on sofia-sip NUA) to work with sipgate.de, with TLS enabled. The problem is that Sipgate rejects registration with Contact headers with sips: URLs. Unfortunately, that completely breaks Contact URLs generated with natify enabled.

So far, the only workaround I've found is to prepare the contact header myself and add

SIPTAG_CONTACT(SIP_NONE),
SIPTAG_CONTACT_STR(contact_header),

to the relevant calls: nua_register() (but I usually don't have the required information there yet), nua_authenticate() (after an "unauthorized" register response), nua_invite().

I can get the necessary information for NAT handling from the Via header in the "unauthorized" response, essentially replicating sip_contact_string_from_via() with the modification that TLS transport does not automatically produce a sips: schema.

Code duplication aside, the problem is that after setting SIPTAG_CONTACT_STR in the authenticate step the incorrect Contact headers keep showing up in responses to keepalive OPTIONS requests by the server, and then the registration expires quickly. If I already have the Contact URL when calling nua_register() (e.g. on reconnect, assuming the external IP of the NAT didn't change) and set it there, the Contact headers are correct.

So my questions are:

• What's the recommended way to override the Contact header in a stable way? Is there a way to override the default in NUA, or a standard-compliant way to probe before register?
• Could sip_contact_string_from_via() be fixed not to force sips: with transport=tls? I'm not sure if always using sip: might cause problems for other use cases, if yes, maybe a flag would be possible?