"Trying to change the date of an appointment ."

I have been trying to change the date of a physiotherapy appointment since last Wednesday, when I realised that I had double booked myself. For the last four working days I have tried to telephone the number I have been given 02033158404 and on three occasions I have been told I am 4th in the queue and then waited up to 10 mins on landline to be told I am 3rd in the queue. This morning the line is engaged. The point I would like to make is that I find it very troubling to see that so many appointments are missed but I think one reason might well be it is so difficult to get through to either cancel or change an appointment. Why is it not possible to leave a message if after 5 mins there is no answer. Secondly why is it not possible to reply to the reminder text service which I think is excellent. I do not think either of these two approaches would have significant cost and might save money.