How much time do phone confirmations cost?
Appointments in the month times the minutes per call. With 300 appointments and 3 minutes per call, that's 15 hours a month of front desk time on the phone, not counting people who don't pick up and need another call.
What can happen on its own?
The reminder and the request to confirm. Whoever confirms in the app doesn't need a call. Whoever doesn't confirm goes to the front desk queue, and only those need a call.
What about a canceled slot?
It goes to the waitlist, and the first person who accepts gets it. Two people can't get the same slot: the second one is told it was already taken.