ATOS must by law give you 7 days written notice if they want to medically assess you.
Legally, this should be in writing, but these days it's not uncommon for ATOS health care professionals to contact claimants by ' phone a couple of days before they wish to call.
All claimants are perfectly within their rights to refuse a medical if it's not in writing and at least 7 days notice.
You were well within your rights to refuse.
Wishing you a very Happy Birthday
Jim