medical debts

I have recently retired due to a serious medical condition. I was given a full disability medical retirement. Last spring my pain management doctor requested that I have an MRI performed by a doctor's imaging service. At the time we had United Health Care. When my wife and I signed in for the test, we asked very clearly as to whether they accepted United Health Care and we were told yes and that we would have a small deductible to pay. A few weeks later, we received a bill from the imaging service saying that we owed them $1,265.00 due to United Health care saying that the imaging service was not an in-network provider; but an out-of-network provider. I contacted the imaging service immediatly thereafter and was given an apology and was told that they would review my case. I did not hear back from them over the past 10 months and recieved a letter from a collection agency saying the i owed the $1262.00 and had 30 days to pay this to keep from having this turned over to credit agencies of which would effect my credit. Do I have to pay this within 30 days or can I make payments over a year and not have my credit effected?

Thanks!