"No-one answers the telephone or responds to..."

Anything else?

My son attends the Orthodontics Department and had an appointment on 5th November with a follow-up due on 27th November. On Monday 11th at three o'clock, a man left a message on our home phone requesting that he attend an extra appointment at 10:45 on Wednesday 13th. By the time that I picked up the message it was too late to ring to rearrange. After numerous attempts on Tuesday (phone constantly engaged, being put on hold etc) I was eventually connected to an answerphone service so left a message (with three alternative numbers) asking someone to contact me. Absolutely no response. I tried again this morning and evenually was able to leave another message. Still no response. I have also completed the "rearrange / cancel appointment" form on the website. I have requested that someone ring me to discuss alternative dates for this extra appointment. It is no good being allocated another slot as it is impossible to get through to the department to change it. What do I have to do to get someone to ring me?