To answer this question effectively we need to give it some historical context.

Prior to 2004, Telkom was the only legal operator of fixed line voice. When they advertised free minutes or inclusive minutes all fixed line calls were on-net. (On-net means calling from the same network you are calling to, ie Telkom to Telkom.)

To further understand what on-net means, consider a deal/offer from Vodacom or MTN that states on-net, it means Vodacom to Vodacom only. If a Vodacom number (082, 072) has been ported to MTN for example, the customer would pay the standard Vodacom rate and not the special on-net rate because it is not on-net, it is terminating on the MTN network.

Since 2004 the number of licensed fixed line operators has increased dramatically. There are approximately 400 I-ECNS and ECNS license holders who are legally allowed to operate a fixed line network. More than 50 of these are currently active and operational which has created competition and choice in the market place. All these providers have their own geographic number ranges including 021, 011, 031, etc. Geographic number portability has been available to customers of these operators since 2010.  

When you place call from a one network to another, ie Telkom to Neotel, there is an ICASA (Independent Communications Authority of South Africa) regulated fee that Neotel charges Telkom for terminating the call on their network. These fees are controlled and regulated by ICASA and not the operators.

More about ICASA can be found here

Anyone can check the network a number is on by using this publicly available database query, which is made available by the jointly owned (by Vodacom, Telkom, MTN and Cell C) Number Portability Company here

Now to answer the question

Telkom residential plans such as their closer and unlimited calling plans all refer to on-net fixed line calls as can be seen in the opening paragraph of Telkom’s terms and conditions.

“All fixed line calls (including surfing minutes in Plan 1 and 2) must originate and terminate on Telkom's network to qualify for the benefits of Telkom Calling Plans or the optional Internet add-on. Determine if a telephone number is an Off-net number”

Source -

Telkom also makes a Verify Your Number option available for their customers to check if the geographic number is on their network. See here

It is important to note that all charges a customer pays their service provider (Telkom, Vodacom, Neotel, ECN, Vox, Mweb etc) are dictated by their service provider and not the terminating network owner. Any fees charged between service providers are regulated by ICASA.

The perception that the Telkom closer plan is misleading customers into believing all geographic landline numbers such as 021 or 011 are included in their free minutes is a historical one. If Telkom wanted to include all landline networks within their free minutes they could, it is entirely their prerogative.

We do hope this clarifies any confusion, for a country to have multiple service providers and multiple network providers is essential for competition and a free and fair market.