This is the direct link to MTN's subscriber terms and conditions
https://mycontract.mtn.co.za/subscriber/tc/susbcriberTC.htm?lang=en
It appears to say the contract will continue indefinitely unless cancelled by the contract holder or by MTN in writing:
CONTRACT STARTING DATE, RENEWAL AND CANCELLATION
2.1. This Contract will start on the date of Activation of a SIM Card issued to you by MTN SP ("commencement date") and will, subject to clause 7 below, continue for the Initial Contract Period. At the end of the Initial Contract Period this Contract will continue automatically on a month to month basis, unless renewed by you in writing, or cancelled:
2.1.1. by you, at the end of the Initial Contract Period or a Contract Renewal Period, as the case may be, by giving to MTN SP a written notice (not via SMS) of cancellation not less than 1 (one) calendar month and not more than 3 (three) calendar months before the end of the Initial Contract Period or Contract Renewal Period, as the case may be; or
2.1.2. by MTN SP, if it chooses to do so, immediately on written notice to you; or
2.1.3. by MTN SP, immediately on written notice to you, if the sub-license issued by the Operator in terms of which MTN SP is authorised and empowered to give you access to the Network Services, is cancelled for whatever reason.