Airtime disappearing

I have a vodacom simcard in a spare phone of mine. I load airtime monthly to keep the nr active. The phone is off, I never phone or sms from it or use any apps at all. Its out away in a safe place for when it is urgently needed. I put the phone on last night, checked my balance, lo and behold there is 0.00. I send an email to customer care, no reply and yet, they advertise that they will get back to you asap within 24hours and nothing, no email reply no phone call. I just want my airtime back on my phone, it is NOT a contract and is not being used.