MTN customers in parts of the Free State and Western Cape were left without service on Thursday morning.
According to a spokesperson, the downtime was due to "urgently planned repair work".
MTN teams are busy restoring the network, which should be fully online around 09:20 on Thursday morning.
"Our apologies to our customers in those areas who were affected. The work was urgently required to ensure we can maintain the high quality performance our customers receive on our network," MTN said.
Outage tracker Downdetector shows a spike in reported connection problems around 08:00 on Thursday morning:
UPDATE: Late morning on Thursday, MTN confirmed that its service was fully restored.