Visual voicemail: does it work with WiFi?

[Mary100]

Hi, I have an iPhone 6s Plus and o2 has apparently given me the Visual Voicemail 'bolt on'.  I asked them to also give me iData but they said it's not necessary.  

The reason why I want visual voicemail is so that if I travel abroad, I want to always keep my phone in flight mode (and switch data roaming off) so that I don't incur huge roaming costs, and I just want to use WiFi.  

Does visual voicemail work with WiFi only? That is, will it be possible to see the little red dot indicating that someone has left me a voicemail by only having WiFi on?

At the moment when I turn my phone to flight mode, there is a note that the top of my voice stating "visual voicemail is currently unavailale".  Does that mean it's unavailable because flight mode is on, or does it mean that o2's visual voicemail is currently 'broken'?

I would be very grateful for any help.  Thank you!  I posted this to the "Apple" board, would I get a better response posting it to another board?

Thanks!

[jhelp]

I have found this to. I am not sure if the same happens on EE m. What I don’t understand is visual doesn’t work on 02 WiFi call so still on the 02 network. Can o2 check this as I thought visual worked on WiFi from EE experience.

[gmarkj]

It does, but o2 need to tell their system you are using an iPhone @jhelp.
Call customer services and ask them to add the idata bolt on to your account (this is not something you can do yourself).
Once enabled, you need to reboot your phone and it will start working.
The best number to call customer services on is 0800 587 4005.