The mobile phone installment payment was paid off last month, but I still received a SMS notification this month. What happened?

There are three situations: first, the service is delayed, so even if you have paid, you will still receive the information.

Second, you didn't fully complete the payment, that is, you may need to sign some documents, but you didn't sign them, so you still received the information.

Third, you may be cheated by the place where you sell mobile phones.

If the suggestion is not clear, you'd better take the initiative to ask the local authorities to confirm the situation.