Most South Africans know that Telkom is already busy phasing out their copper lines (which ADSL and landline phones operate on). What Telkom usually does is immediately offer a LTE based handset with the same number which operates like a landline phone.
But if you have ADSL you'll want to move to ideally fibre or if not available then an LTE-A Internet service. On both the fibre and the LTE-A service you can operate a Voice-Over-IP phone service (you need to consider if your cellphone is not just good enough to use then you would not worry with this VoIP service). But if you want to keep your old Telkom landline number to use on VoIP then you must port that number while you still own it (not after you have cancelled your landline).
So to do this, you register with a VoIP provider and arrange through them to do the porting. It takes a few days and then your old number is available for you on your new service.
I went with Freshphone after looking around as there were no registration, monthly costs or contract, just a top up when you need to replenish credits for outgoing calls (nothing extra for voicemail or itemised billing). I did pay a small once-off fee for the number porting. Calls to others using Freshphone are free. I bought a Yealink VoIP phone handset so it all operates like a landline phone, but I can also use the app on my mobile phone and then I'd receive calls even when away from home if my mobile has Internet connectivity. Up to now an 087 (or similar number) is free but I see they now offer some geographic numbers for a once-off R25 (021, 010, 031, 012, etc).
But shop around as there may now be better or newer providers to consider.
See VOIP Phone | Fibre to the Home & Fixed LTE Internet Calling | Fresh PHONE
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