Installment placed on a currently owned phone.

If your phone did not have a payment on it, they may have used your line to purchase a new phone for themselves. I believe it's called a cross upgrade.

Can you provide more details?  No one can just put an installment plan on a phone that’s already paid off.  Either you buy the phone on an installment plan, or you buy it paying full price at the time of purchase.

As @SoloTX  wrote. Probably upgraded your line.  Talk to the others on your plan