I have an unpaid balance on my phone but it's for a purchase I never made

Typically you have to pay the balance and request a refund.

This support article is Recommended by Apple as the preferred* resource for identifying payments you may see on a bill.

➜ If you see 'apple.com/bill' on your billing statement - Apple Support

"If you checked https://reportaproblem.apple.com and your emailed receipts and you're still not sure why you were charged, contact Apple Support" using this link --> https://getsupport.apple.com/?caller=ikb&PGF=PGF51002&category_id=SC0060&symptom_id=21084

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* I have a payment of $2.99 per month what … - Apple Community

Once you have found the item in your purchase history, here's how to request a refund (From: Request a refund for apps or content that you bought from Apple - Apple Support - see the article for more detail.)

1. Sign in to https://reportaproblem.apple.com.

2. Tap or click "I'd like to," then choose "Request a refund."

3. Choose the reason why you want a refund, then choose Next.

4. Choose the app, subscription, or other item, then choose Submit. If you were charged for a subscription that you no longer want, you can also cancel the subscription using the directions in this article --> If you want to cancel a subscription from Apple - Apple Support ].

Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support