What I do when someone phones is to say something like, "the treatment itself takes 45 minutes, but you will need to allow an hour as we will need to go through a consultation to check that you haven't got any medical conditions or take any medications which won't sit well with the treatment". This then gives them the chance to volunteer some information, or ask a question if they are unsure. On occasion this has highlighted a condition needing them to check with their doctor, and they can go ahead and make the appointment but have a chat with their doctor first. Sometimes it has highlighted a condition best treated by someone else, eg a sports injury therapist, and I will suggest someone to them. On the whole people coming for IHM don't seem to have too many serious health issues - its' the reflexology clients who keep presenting the challenges!
I'll be interested to hear what other therapists do to prevent having to turn a client away!
Best wishes