Demand source documentation, call and make sure you got the service, and if so, ask for a letter stating the complete balance with assurance that paying that amount will be considered payment in full. This last part is important because debit collectors bill an amount they figure you'll "just pay". If you do, you've admitted owing whatever they say you owe.
I did get a bill from an out of network guy and he wouldn't negotiate, so he got zip. Later, I had to get a loan on for a house so got the kind of letter I mentioned above, paid it, and the credit report was clean, like nothing happened. So the point is, ignoring it is an option which doesn't preclude later fixing it.
And you can ignore it and not be pestered. Just write a drop dead letter, signature receipt, stating that you will sue them in your local court, a right provided by the act referenced in an earlier post. This saber rattling is often enough for them to find an easier target.