The on-call provider is who is paged when no one is scheduled. That's the clinic owner to start with.
To finish this row, add and verify your cell phone in Account settings: enter it with the country code, and we text a code to make sure the number is yours. You still sign in with your authenticator app.
If you make someone else the default provider, they verify their own cell when they sign in. The row is done once the default provider is active and has a verified cell phone. It is needed before calls work when your clinic holds on-call.