The native integration does not usually allow you to use SIP when on a data connection (at least that's how it works for me, I have to be on wifi). From the ar

The native integration does not usually allow you to use SIP when on a data connection (at least that's how it works for me, I have to be on wifi). From the article, I understand that if the SIM has a data only plan, then there are no restrictions for VoIP.