Vodafone (and many other providers) proudly declare something to the effect of, "There is no extra charge for WiFi calling, the minutes will still count towards your allowance and there won't be a charge for the data against your allowance.".
But if we're not using the cell towers and most of their network (except last mile), and we're providing our own already paid-for data connection... why would WiFi calling count towards _any_ allowance since it incentivises behaviour that reduces costs for telcos?
Is this purely a "They found a way to reduce their costs and still charge us?" or is there more to it?
Not only that, but keep in mind that Wi-Fi Calling (and VoLTE) relies on SIP, so your phone has a full-blown SIP client fully integrated into the system, and the only reason you aren't given access to its settings is to lock you into your carrier, otherwise people will switch to third-party VoIP overnight and carrier monopoly will become a thing of the past.
Its very convenient to use vowifi vs getting a local sim and not receiving calls or SMS on your home number.
Telcos can offer it as they already need the infrastructure to do voice & messages over 4g anyway.
I make free video calls all the time. The gotcha here is that the person on the other end has to run the same software as you. Part of what you pay the wireless carriers for is the universal compatability.
Personally, I like this: https://twin.me/en/