Phone constantly in repairs

We bought a nokia handset from MTN 20 months ago on contract. The phone was problematic and we were advised to book it in for repairs. the phone was returned to us 2 months later and a week aftyer that we had to send it back to the store and again it was booked for repairs. In the 20 months that we have had the comntract my dad has only been able to use the ophone for 3 months as it is always in repairs. MTN has not provided an alternate phone nor have they offered to provide my dad with a new phone as there are obviously serious problems with the current one. We have basically been paying R400 for an abscent phone.