Tell them you will report them if they call you before 8am and after 9pm in your time zone. It's illegal.

Check out the Federal Fair Debt Collections Practices Act.
It's illegal for them to call before 8am and after 9pm in your time zone. It IS LEGAL for them to call on holidays and weekends.
Check your past due bills. It's likely a collector of some sort.

File a complaint through all of the credit bureaus. They will be required to show you what they are trying to collect.
Check your payments have ALL PROCESSED. One may have been misapplied to a wrong account or had a problem processing. I talk to people ALL of the time who swear they've paid, only to find that the payment they mailed got lost, the bank didn't process it because of some small error, whatever.
You NEED to check that ALL of the PAYMENTS YOU'VE made have FULLY PROCESSED.
It may well be an error that can easily be fixed.
If it's YOUR error, you will have to tolerate the negative impact on your credit report. If it's the company's error, THEY will need to report the error to the credit bureau and repair your credit record.
By the way, it is legal for them to call you any day of the week, including holidays and Sundays, from 8am to 9pm in your time zone.
Easiest remedy is to check your payment records.

It sounds like someone gave them a fake phone number and it happens to be yours. If you call the company and tell them they have the wrong phone number they will likely ask you if you know who they're trying to contact. You can tell them you don't and to remove your number from their records.
It usually takes less than 2 minutes once you get connected to a real person.

It's harassment when debt collectors are calling outdated numbers. I for one can testify to this exact thing since I have received calls for people that I never heard of.