Long outstanding Repair

On the 20 November 2017 I took my Samsung J5 Prime to MTN Store in Woodmead due to the phone restart its self every time I play music .On 12 of December I went to store to chech on my phone if it was fixed but I was told that the phone is still on repair and they will do follow up and they will call me if there is new update but how ever nothing no one called me and on the 18 December I went to store n to chech on my phone if it was fixed but i was told that the company that's busy with repairs is busy with stock take they won't release the phone until they are done to add on my frustration because I'm paying this contract this is poor service and they keep on promising to call me with regards to this matter and On the 22 December 2017 i went to the store again to check my phone but i was told that my phone was with Samsung repair d they will do follow up and they will call me and tje store has not call me with regard to my device more frustration because this poor service from MTN