No signal despite various calls to call centre

I have taken out two contracts on my name for both my mom and dad earlier this year. At various points have I reported that their signal goes away for long periods of time whereafter I was told that it will be looked into and that they must switch to Vodacom as carrier in the meantime. This however jumps back to Cell C automatically after a while. Sometimes one of them will have a signal, with the other's phone is next to them, and then the latter will have no signal. Surely there must be a way to fix this/look into this without them having to switch the carrier to Vodacom every five minutes? I am worried that if they have to get hold of me for emergencies they will have no signal. Please can someone look into this and get back to me, since the calls to the call centre has been fruitless up to now, and I am very worried about them not being able to make calls most of the time.