JI'm in the same boat. The biggest issue IMO is that the only SMS provider that currently works with Zitadel is Twilio. Twilio requires the country code to be part of the number, eg, +1 for US. The UI does not indicate any format of how the number should b entered and when users enter a number a number without the country code there is no failure mode other than they will never receive a code.
Rhey @Jeff D @callumboase thanks for the tip, I am raising this internally with the design team so that this could be noted.