Viber is VoIP client for iPhone which has racked up one million downloads in the 3 days after launch (according to 3G.co.uk).
It allows you to make free calls to other Viber users. Is how is it different?
- Well your ID is your mobile number - which eliminates registration / authentication, making adoption easier. (This is the most novel part I think)
- Once installed, Viber automatically scans contact list in your mobile phone and highlight the users that have Viber, so you know who can you call for free. (Truphone does this too, I believe)
- Viber runs automatically whenever your mobile is turned on.
Effectively, it is an easy way to substitute your voice minutes by using data package. Given that adoption and substitution is so easy, I would anticipate a reaction from mobile operators.