Escalating billing

After 14 years of paying R3800 p/m on my 5 phones.In April my bill jumped to R 7400. I immediatelly phone Vodacom and they said I am using to much data on my daughter's phone. We spoke with Vodacom and changed all the settings on her Samsung S5, I paid the bill. May comes and it escalates to R10200 . We phoned again and the same was said \ using to much data". We went through all the settings and the lady said that she can't understand as all the settings were fine. I was told to pay R 4000 and the balance will be investigated. We have wifi at work&home. She uses her tablet as a hotspot at her flat. She is very seldom on her phone for wattsup and sms'e as she has computer to her disposal.Nobody at Vodacom can tell me what data she has been using"