Earlier quoted context omitted.
Accessible would be the mobile networks allowing people to call overseas for $0.01 per minute. I would not dare try to call an international number via ATT/Verizon/T-Mobile outside of Canada/Mexico, because I would expect to pay at least $10 for the call if not more. I do not know where one would get $0.01 per minute international calls from the US, but it is most likely not as easy as an app on your smartphone.
That's on top of paying for the internet itself. You must be young. If international calls are what you are looking for, calling cards and prepaid phones have been easy and cheap since a decade before these messaging apps. Then there was Skype and desktop messaging, and VOIP.
Paying for the internet is not a consideration since people will be paying for the internet anyway.