Fraud shop owner

Friends, I know my installment is fully paid, but still every day and every month I keep getting calls. Now my mobile has been locked, and these people have increased the interest, turning an 18,000 rupee phone into something for which today they are asking me to pay 30,000 rupees. I have already paid 16,000, only 2,000 was left, but still now they are saying I have to pay 14,999 rupees. My point is, never buy a mobile phone on installments.