MTN plays UNO reverse card on me

One morning I found myself unable to gain access to my mobile device. The device unlocks using the password and then immediately locks itself. I booked the device in 3 weeks ago at Pep Home The Bridge. I even showed the fault to the lady assisting and gave her the pin. The phone was booked in to go back to MTN to be repaired. I received a call this morning from PEP Home The Bridge that the phone has returned from repairs. Nothing was done to the device. MTN informed us that they couldn't do anything as the device is locked with a pin. Now I have to sit another 3 weeks for them. I do not understand? What is even the point to send it back If they sending the device back to me for the same reason I am sending it to you.