Bad service and carelessness with customer stuff

I took my girlfriends Sony tablet to MTN for screen repair to replace the cracked screen and they told me to leave everything that came with the tablet since it's a contract tablet including the sim card as well. I left everything with them and they took it to repair center. They sent and sms with the quotation for repairs and on their contract it does state that if you are not willing to fix your device you may request it back however there is a fee that you need to pay. So yes we requested the tablet back as it was too expensive to repair and we payed the required fee to take back the tablet. They first said we wont get it back because it had stayed too long and then all of a sudden they call and told us the tablet is ready for collection. When my girlfriend went to collect it she found that it was a wrong tablet that they brought. They took it back and told her that they will contact her but they didn't. She phoned to follow up and they said her tablet is ready for collection, but when she got there the charger and the sim card were missing. Yesterday we went back again to collect it, the charger is there but no sim card and Mary Ndebele denies that I left the sim card.