Someone paid my cell phone bill this month and wasn't me or DH. I called to make my payment and she said, "Ma'am, your payment has already been made this month."

Huh????
She said she couldn't tell me WHO paid it. Just that it had been paid already.
I KNOW I didn't pay it. DH swears he didn't pay it. It has not been debited from our bank account. I don't have it set up on automatic bill pay.
I have no idea how anyone other than DH or myself could have paid it because no one else knows the 4 digit passcode we have to use or anything like that.
I am very confused.