8ta / Telkom Mobile
8ta Not working as should, signals almost 0

I do not understand how my 10GB 8ta can work fine today, and then tomorrow it says SIM not valid or registered, then 2 days later it works again. I have left 3 messages with 8ta, but no response what so ever, I still have more than 7GB left for the month and
I would like to use it.