Not a happy customer

in the beginning of july i switched over to cell c.ever since i have changed networks the following has been happening. My phone will be on,battery fully charged,mobile data is on with a full Cell c network coverage. Customers,friends and family will start calling me over whatsapp because when they make a call to my number it will go straight to voicemail. I will reset my phone and receive sms's about notifications of missed calls. So in other words i never know if i am online or not????? I have went into the cell c shop where i was advised to call customer care.this i have done more than 10 times.Either i will be told that the problem has been solved and i will i rate the service or lately...We will get back to you! This problem is still happening so i would just like to know what the problem is,if it can be fixed and or if i should change networks again??? Regards.