Well that seems accurate. Cell C's coverage doesn't work. I am tearing my hear out on a daily basis. Reception just went to hell. The number of times I get "Emergency Calls Only" message is insane. Living in Cape Town. My contract is up in July. Whether I like to support the underdog or not, I have to move because my phone literally doesn't work. The only reason I survive is because fibre at my home - otherwise my phone is a paperweight.
The "emergency calls only" issue must be because of loadshedding either in your area, or adjacent to you.
It's not much different - at times - on MTN for voice calls ; we don't get the same message, but outgoing calls simply don't go through at times, so need to keep retrying until they do.
That said, still sticking with MTN (particularly for making calls), because we regularly get an option to double our minutes for the same price, on prepaid.
Has happened the last 2 months in a row, and a few times prior since switch over from Cell C.
For R100, there's (often) an all-net offer of 480 minutes for 30 days, along with the standard 240 minutes.
So try testing it out before your Cell C contract ends.
Vodacom is the most reliable connection (here), but only top up occasionally per SIM for backup, even though they're our main prepaid numbers.