Owing Money on my Apple product

Hey there! I got my iPhone X the year it came out, and i was doing the next plan with AT&T. Unfortunately I was out for ten months, and I didn’t have contact with my service provider nor have a way to pay my monthly bill, so whatever owed on my iPhone ended up just being the full amount people usually pay for the phone $1000. I want to upgrade my phone, but I can’t because I owe this money, I was wondering who do I talk to to pay this debt and is there any way that I can pay in payments so that I can get a new phone