When a Telkom SIM is out of reach of the core Telkom network, it will roam locally (within South Africa only) on the Vodacom SA network.
This roaming is seamless and will not require device restarts. Roaming is on 2G + 3G + 4G (LTE).
Note: Certain Telkom data bundles ("LTE" bundles) are not enabled for roaming on Vodacom - they are Telkom core network only. AnyNet and FreeMe bundles will also roam on Vodacom.
Specific requirements for 2G devices:
"Roaming should be seamless for customers on 3G and 4G devices. However, those using 2G devices must first connect manually to Vodacom’s network by connecting to the network ID “TelkomSA-R” (the name, or PLMN, given to Vodacom’s network for roaming Telkom users). This is because these handsets don’t support the MOCN roaming technology out the box. Once 2G customers have done this, they will be switched back to the Telkom network and the next time they move into an area where Telkom coverage is poor or non-existent, their phones will automatically roam onto Vodacom. Though any Telkom user can manually select “TelkomSA-R” in their phone’s network settings, they will almost immediately be bounced back to Telkom’s own network provided they are in a Telkom coverage area.
“It’s very important for 2G customers to have seamless handover, too,” Apleni said. “The customer has to go through the manual setting to clear barring (of Vodacom’s network). Then it will register Vodacom as an acknowledged roaming partner. This is a once-off process.”
More details on the roaming agreement can be found here: https://techcentral.co.za/telkom-roaming-gets-a-big-shake-up-this-is-how-it-works/90693/
More details on how local roaming works on Vodacom here: https://mybroadband.co.za/news/cellular/313815-why-your-mobile-data-doesnt-work-on-telkomsa-r.html