I've been through this voluntarily a while back so can explain the process a bit. Many will just ditch their landline number and use their mobile phone, but what if you want to keep that landline number you've had for 30+ years? Telkom may offer you a wireless LTE phone if you only want a telephone, but those are not really very practical and cost you per month still.
Generally where Telkom is discontinuing the ADSL Internet service (which needs the copper line to work) you would switch over to either fibre or fixed LTE Internet access (neither require a copper line subscription so you save that R200pm). Both of these will allow a Voice-Over-IP (VOIP) service to work just fine (you'll make and receive phone calls over your Internet link). In South Africa you can port your existing landline number over to a VOIP service provider, BUT you need to port it while you still have that landline number active as you need to show proof of "ownership". Once its ported you can cancel the original landline service. I did manage to port my pre-paid Telkom number, but this meant I had to go to Telkom's shop to obtain an account "proof of ownership".
I went with FreshPhone as my VOIP provider mainly because they can port easily, there is no monthly subscription cost, Freshphone to Freshphone calls are free, and I could connect either using an app on my mobile phone (if I was travelling) or I could use my WiFi-based Yaelink VOIP phone in the house.
One other thing that may be using that copper phone line is many alarm systems, and for that I just asked my alarm company to install a wireless communicator which works just fine and has an added bonus that no-one can cut a phone line to disable it (your fibre link could be cut if it's an overhead one, but fixed LTE is not subject to this problem).
In summary then you can:
1. Do nothing - use your mobile phone number.
2. Take a Telkom wireless LTE phone and continue paying per month. Will have to still take additional Internet service.
3. Port phone number over to a VOIP service on your fibre or LTE Internet link. Cannot port phone number later, but can take a new phone number allocated.