Hi Tech Medic!
This is a good question and intuitively feels like it should be possible but there are a few factors that keep this feature off the table. SMS reminders cannot be a required field due to privacy and carrier regulations, you are not guaranteed to receive an invitee's phone number when requesting invitees to opt in for text messages.
If you need their phone number to contact them later, we recommend adding a second, required invitee questions asking for the best number where they reached during the day, or something to that effect. This slight distinction indicates that this number could be a work line rather than the mobile phone number for text reminders, making the double-entry appear less burdensome to your invitee! Sorry this isn’t the answer you are probably looking for but I hope this helps!
Thanks for the explanation. We’ll figure out if a second request for phone number - like you suggested, makes sense. Thanks!