insurance/collection agency problem

What is the name of your state?

california

i have a problem relating to a hospital bill. i had a hard time getting the hospital to send the bill to the correct address at the insurance company. it required many calls over a period of months before the hospital sent the bills to the correct insurance office. in the meantime, the hospital sent the account to a collection agency. eventually, they sent the bill to the correct address for my insurance agency, and my insurance agency paid their portion under the PPO agreement. i also have a copay amount, and there was a large portion of the bill which was disallowed under the PPO agreement. my problem now is that the collection agency is charging interest on the entire original bill, including the portion disallowed under the PPO agreement. since neither i nor my insurance company was liable for this amount under the PPO agreement, how can they charge interest for this money? is it legal to charge interest on money that was never owed? is there anything i can do about this? thanks for any advice.