Bajara:
Call the hospital and find out what you owe and whether the collection agency is actually working on their behalf.

If you want to paly by the rules and protect your interests.

1. send them a registered letter or fax mentioning that you need to know the real creditors, on whose behalf the credit agency is working. Mention that you wish to resolve all the dues in an amicable manner. Keep all the documentation.
2. Ask them to correspond with you only in letter/mail and stop making calls. Choosing the way you wish to be communicated is your right.
3. If they are not able to provide the real creditor, it is most likely a scam and just ignore them. If they continue harrassing you, tell them that you will be filing a police report for scam/fraud.
4. If they are genuine collection agency and will not provide the real creditor, then you write to them telling them that you will not entertain further correspondences until the information is made available to you. Again, mention that you will proceed with legal action, should your credit rating suffer because of their stupidity.

Good luck... If I were you, I would not lose any sleep over it.