Phone auth only in transactin model?

When adding Sign to a VIP is necessary to choose between NUL licenses model or transactions model, but both at the same VIP are not allowed. When the verification in two steps is selected, veryfication via telephone is an option for business and Enterprise licenses, but: only in transaction model? Because every phone validation uses a phone transaction and, as far as I know, this transaction model can´t work with NUL licenses.