In 2010, free calls from a mobile phone still felt newsworthy. I wrote about Viber: an internet connection let you talk to another user without using ordinary mobile call minutes. The app used a phone number as an identifier, so people did not have to exchange a separate username first.
Viber still exists. Its current setup guide says you use a phone number to activate the app and can call or message other Viber users. The old description of a new iPhone app and features still in development belongs to 2010, however, not to a guide for today.
What is actually free?
An app-to-app call does not require ordinary call minutes, but it does use an internet connection. On Wi-Fi it need not consume mobile data; away from Wi-Fi it may count against your data plan, and roaming terms can differ. Calling a regular number outside Viber is another service and may be charged. Check its terms before relying on it.
Convenience and reach
Using phone numbers makes it easier to find people you already know. At the same time, synchronizing contacts deserves a conscious choice because you share information with a service. Review permissions and privacy settings on your phone and in the app. For business contacts, decide which account and number you want to use.
The value of my original tip lies in the shift Viber represented: calls became an internet service layered on top of the phone. That is now familiar. Choose a calling app according to the people you need to reach, the costs involved and the control you want over your contacts.