I may be wrong but I don't think this is part of the feature of the appointment calendar. You'd need some kind of gateway/exchange for that and it wouldn't be in Lacerte.
If you need SMS reminder for appointment, you may want to consider other solutions like ZipWhip.
Still an AllStar
Lacerte appointments are archaic. They work only within Lacerte.
This group does some fancy code editing to do things that I could only dream of. If you have someone with those skills they could maybe type Appointments to something outside Lacerte. https://proconnect.intuit.com/community/lacerte-sdk-group/gp-p/501