Visual voicemail only on mobile data? not wifi?

Hi, got a question. Whenever I get a voicemail, I don't receive any notifications from fizz's visual voicemail app. Only once I've checked the voicemail by actually calling that I then receive the following notification from the app: "This operation can only be performed if the Mobile Data network is available".

I am connected via wifi but mobile data is enabled/available at all times with strong signal.

If I disable wifi, the app will then work normally. As soon as I get a voicemail, I will immediately get a proper notification from the app saying that I have a new message. Turn wifi back on, and then no notifications until I actually call the voicemail the "old way".

I asked support about this and they say that it's normal, that I have to be exclusively connected thru the mobile data for it to work. If I am on wifi, it won't work.

Seems rather strange. I mean, most of us are on wifi when at home, so the app wouldn't work whenever we are at home??? o.O

It's no big deal for me but I just find this limitation a bit strange. What do you all think?