They only charge if you don't cancel 48 hours in advance. I had to cancel the day of due to my daughter's illness and they didn't charge me but I had a doctor's note for her. I sometimes get voice mail but the receptionist is very diligent about returning phone calls from what I've experienced.
I've never had an issue with getting a person on the phone. You should be able to leave a message with the receptionist, and she will get back to you. The reason they hold a credit card number is just in case you cancel the appointment after it has been made.
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