dxbpilot,
I posted an identical thread in the Medical forum about a week ago - in exactly the same boat as you.
After many exasperating phone calls from Canada, and getting the same response as you are, I elected a family member in the UK to do the calling for me. A call was placed at bang on 0900 BST and by amazing luck they answered and said medical was booked on my behalf. Apparently if you leave a message, they will call you back wherever you are in the world (hmmm...with time change and the organisation required to do all this, I am personally doubtful)
Your best bet is to try and get someone in the UK to leave a message and get the CAA to call them back during UK working hours. Nice and simple for them. Otherwise, its pot luck really - leave a message, keep that mobile phone on all hours and hope they call. Crap system really isn't in when a medical costs anything between £260-£500!