Even where its not required, it can be a lot cheaper (e.g. for international calls) and has features (conference calls) not offered by consumer phones.
Whatsapp's success has partly been driven by telecoms companies over-charging.
Whatsapp's success has partly been driven by telecoms companies over-charging.
There is no subsidy, its telecoms companies making a ridiculously high margin (aimed by regulators in some countries) on some services.
I don't see the issue. I do the exact same. Want to talk to me? Use this app or we can't. There never can be any network effect if nobody start doing this.
They do it because they can and its not important to most people.