Insufficient Airtime

Most of the time when I male a call, the phone get dropped. The message that Im getting is that I have insufficient airtime. This happens even if I have about R30 airtime and data in my SIM. I called the call center (173) and I was not satisfied with the answer. The call center agent says that the airtime is reserved for the applications that are running. And since about the past 3-4 months, my data is being disappearing. I uninstall some applications and block background data in all applications excluding Whatsapp and Opera browsers, but the data is still disappearing. I would like MTN to investigate this.