We're technologically well equipped: ipads, Macs (thus Router, internet etc), 'Smart' phones (although only Pay as You go, not with contracts).
Our phones, calls & text only, suit us fine. We don't pay O2 much; we keep a BT landline going, although more out of tradition than anything else.
But here is our current problem: we'd like to join WhatsApp, because many of our neighbours use it to pass on details of local dodgy characters, etc., and that seems a useful network.
But to be a 'proper' WhatsApp user you have to put it on your phone. Which has to be connected to the internet. Now, if we just turn our 'data' on our Samsung phones to 'on' while we're in the house, the phones are connected via our Router.
But do we get charged for this usage by O2? Asking O2 is a hopeless task - all they want to do is flog you a contract.
If you've got 'data' turned on, the device in your hand isn't really a phone at all (because it's not connecting to any phone system). It's effectively a 'mini computer' which, just like your desktop, laptop, iPad or whatever, is connected to the internet through your router. (i.e. it would still connect to Whatsapp if O2 switched all their masts off).
if you are using your phone on your own wifi via the router you do not have to pay for it. if you wanted to make doubly sure, turn your mobile data off, the there is no chance of having to pay
my iphone "prefers" a router connection to a data connection. I don't have to turn off data when I am indoors, everything except actual phone use goes through the router (hub).