I tried posting this to r/personalfinance once before and got downvoted to oblivion by people who just assumed I was an idiot. However, the following is actual useful, thoughtful advice (in the US):

IGNORE THE FIRST MEDICAL BILL.

Don't pay it, don't call and negotiate, don't stress about it. Open it, read it, set it in your tickler file to come back and look at it next month.

Here's how it works:

1. Visit the doctor, pay any amount due at the end of the visit.
   
2. The doctor sends a 1st bill to the insurance company.
   
3. The insurance company refuses to cover a few things, offers small discounts on others, and agrees to cover portions or all of some of the charges. Remember that the Insurer's goal is to pay as little money to the doctor as possible.
   
4. The doctor adds the Insurer's discounts and coverages to the 1st bill and then sends it to YOU, the patient. Remember that the doctor/medical practice's goal is to get as much money as possible, and they don't give a fuck who pays it.
   
5. YOU get the bill. If you're smart you do nothing. Otherwise, you promptly pay the bill thinking, these are professionals here, they're honest, they know what they're doing, they wouldn't intentionally screw me over would they? They would. Let's assume you're smart and you don't pay the bill.
   
6. The doctor sends a 2nd draft of the bill to the insurer. They may have adjusted their billing codes or their billed amounts to reflect what the insurer covers or to match their network's agreement with that insurer.
   
7. The insurer updates the items covered, not covered and the applicable discount.
   
8. The doctor updates the 2nd bill to reflect the insurer's updated coverage.
   
9. The doctor sends the 2nd bill to you.
   
10. You open the bill and notice that it's substantially smaller. Some charges disappeared, some were reduced, some received larger discounts or higher insurer covered amounts. You promptly pay the 2nd bill.
    

I've been doing this for a decade now. Never had a bill show up on a credit report, never had one go to collections. A few notes:

• If I don't get a second bill, then I'll call the provider and ask if I have a balance due.
  
• If I know from past experience that the 1st bill is correct, I just pay it. The kids get annual check-ups and I have to visit a specialist every 12-24 months. The bills are generally 1-2 line items and the provider's already know how to bill it right the first time. So I just pay it.
  
  • If the bill is coming from a hospital, I'll wait until at least the 3rd bill to pay. Again this is based on experience that a hospital usually takes at least 3 tries to get it right. I've payed 3rd notice hospital bills and then still received additional bills later because the fucking anesthesiologist still hadn't submitted their charges 90 days after the visit. Again, never had a bill go to collections or ding my credit report.