Airtime Dissapeared

I NORMALLY RECHARGE MY PHONE WITH R1100 AND R800 EVERY MONTH AND THIS MONTH MY AIRTIME DISSPEARED I REALISED THAT YESTERDAY WHEN I WAS SENDING MY MOM AN SMS MY PHONE HAD 0.00 BALANCE. I HAVE WIFI AT HOME THAT I USE FOR INTERNET AND WHEN I PHONED VODACOM I WAS TOLD THAT MY AIRTIME WAS FINISHED BECAUSE OF INTERNET USAGE I ASKED THEM FOR A STATEMENT OF PROOF AND I WAS TOLD THEY CANT SEND ME THE PROOF HOW AM I GOING TO BELIEVE THAT IF I CAN BE SENT PROOF IT WITHIN MY RIGHT TO BE SENT PRROF IF I DONT BELIEVE WHAT THEY ARE SAYING IN THEIR CUTOMER SERVICE. I INFORMED THEM MY PHONE WAS LOSING NETWORK YESTERDAY AND THAT WAS NOT EXPLAINED. MY AIRTIME LASTS ME FOR THE WHOLE MONTH AND EVEN REMAINS TO THE FOLLOWING MONTH.