"Unable to contact to rearrange appointment"

I had an appointment booked for my Son at the enuresis clinic in Oswestry, but I ended up having to work that day. I tried to ring to rearrange the appointment and let them know that we would not be able to attend, but the number provided on the appointment letter just returned with "this voicemail inbox is full". 

I then went to the website suggested on the letter and found a number to ring which was just a general number for Shropshire health teams. They also could not help but suggested that I ring the centre where the enuresis clinic was being held. They put me through to the original number that I had called, to no avail. 

My letter says that should we miss this appointment, no further appointments will be given. I haven't been able to contact anyone, nor have I heard from anyone since.