@dimpase @dimpase I've asked the Conversations.im support channel, waiting for their response.
I know people in some countries (e.g. Brazil) don't get Quicksy OTPs because of Quicksy's SMS provider...they reported success with @prav (a fork of Quicksy, which uses a different SMS provider; they're also in the process of registering as a co-op). Prav is not yet in the Play Store, though, so it needs F-Droid for now.
But it might be that an SIP number won't work for either one...
@contrapunctus @prav one should have a option of auth via a Signal or WhatsUp message; or a voice call. SMSs are always iffy, especially international ones.