Sent to collections

I've been a xfinity customer for many years, always paid on time. Now I'm considering moving to another provider because I was taken to collections marked as from Comcast. It already hit my credit report and I never received mail or email or a phone call inregards to anything like it. My statements all show I owe nothing and I even called over the phone to also get confirmation that I owe nothing and have a good standing. I'm tired and scared of how this has happened.