DROPPED CALLS WHEN ON WIFI

I don't know if I am the only one, but when I am connected to Wifi at work (Pretoria west) no calls come through, I first thought it was a network problem, but the problem persisted for a month now. I upgraded my Iphone, changed all settings, even reset my whole network settings. Trying to phone Vodacom, only to hear the number does not exist. Why are they not letting calls through when I am on wifi, I receive Missed call smses but all call get a voicemail response. This is super annoying. Vodacom please contact me asap!