Thieves! Run don't walk.
I had my teeth cleaned here 2 months ago. Everything that was asked was paid. Now suddenly, I see a bill of $50 from nowhere. I guess they just hope you pay it without trying to find out what you are paying for?
The number that they want you to call is a toll free number. It is not to their office. I conducted my business there, not California.
The bill doesn't tell you why and what. It just says, "We have determined that you owe us $50". Please pay it at your earliest. Seriously?
Scammers.