DATA & Airtime

Last night i had 948 Airtime and 45 Megs of data. We have uncapped internet at our house and is all the time on. I spoke to one of the agents actually 2 agents of vodacom and both of them told me the following. When i'm on the wifi at my house I still use my mobile data and my airtime when the data is depleted. So I just want to know how do hell can someone that doesn't have technical background say that to me when your WIFI is on you use the your airtime or your data that is totally bull....... That is why we've got internet at our house!!!!! to get back to the data & airtime i've used from last night 23H00 till now 12H00...and remember that's when my WIFI is on and on full strength is 45 megs of data and over R400 of airtime. I mean how the ..... can that be possible. I previously also had my data that ran out but never did it take that much of my airtime maybe a R20 or so but never in 12 hours over R400. I would like someone to explain to me why because this is *****.