Cut off without warning due to MTN billing mistake

I paid my monthly MTN bill in full on 29 Jan 2020 (R248.43). On 6 Feb 2020 I received a text message saying my service had been suspended and demanding I pay R167.15. No warning, e.g. 'Please pay within 48 hours or we will cut you off'! I tried phoning 135 but couldn't get an answer due to ‘high call volumes’. I paid and then went to the nearest MTN shop, where the assistant identified a billing mistake made by MTN and reconnected me. I've had an MTN account for about 20 years, but I'm seriously considering not renewing it when my contract ends. This incident was a 'last straw' after endless annoyances and bad service.