Since the collection agency is working for the hospital, you need to talk to the person who sent you the e-mail and tell them to make sure the collection agency leaves you alone and you do not want to be harassed by any further phone calls.
DISCLAIMER: David J. McCormick is licensed to practice law in the State of Wisconsin and this answer is being provided for informational purposes only because the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship.
You should send a "cease and desist" letter to the hospital demanding that all future collection efforts be made in writing. Further, keep detailed records of all your communications with the hospital and also copies of any letters you send. Send everything certified mail, return receipt so you have proof that the letters were received. If it does go to an attorney/collection agency, send the same cease and desist letter and also send them a copy of the email you received. If they proceed with litigation, consult with an attorney right away.