Non payment

I had an operation almost a year ago, I received a call from the hospital that the account was not fully settled. Momentum requested a letter of motivation from the doctor which was submitted to them. I phoned numerous times, spoke to numerous people at their call centre. No luck. I requested them to listen to their recordings where they confirmed that it would be paid. I still did not receive any feedback. I am happy at Momentum but this is an account that they are responsible to pay!!! I need a person who knows what they are talking about to contact me and not drop the phone after it hardly rang