please help

i took out a contract last year and i went back to MTN westgate store to give back the device as it was not what i needed and they promised to exchange and they said is hould keep the other devise so long, in december they said i must bring back the device, now when i call the store manager the phone goes straight to voicemail, they had the december to date , theres no communication from the store and no device meaning i have to pay for something i dont have, please rectify this i have to complaints references but still no hep *** and ***