Quest Diagnostics

I went to the Quest Diagnostics lab in Haverhill, MA for a blood test and had to go back and forth for almost two weeks with the medicare code given to them from the doctor's office. The lab finally told me they had the right code and they could do the test with no charges to me. It was going to be covered by medicare.
I told them as long as it is covered go ahead and do the test.
After consulting with my allergist about the test results, a few days later I had to go out of town to work and I was gone for five months. After returning home I receive a letter from a collection agency stating I owe Quest Diagnostics $195.72 for the lab test. No one from Quest ever spoke with me about the bill and I knew nothing about it.
I called my allergist's office and was told I should never have been billed; Quest was given the medicare code and it was covered by medicare.
I can't get in touch with Quest billing office and now there is a collection agency demanding I pay a bill for a test I never should have been charged for. This all started on 3/25/19. I found that medicare paid them $2.94 but I guess they don't like the medicare amount so they are trying to charge me the difference.