There's not really a great alternative to Skype where I can buy an "in" number. Viber is slowly getting there, but they haven't sussed out the business model - I don't want, or need, to spend 20 a month on a set number of minutes. I do need a number people can call me on, and a small credit for outside calling should I need it.
As soon as Viber can do that (and to be fair, they are improving rapidly) then I'll move to Viber. As a bonus, it's not owned by Facebook... (Before you ask, it had end-to-end encryption before WhatsApp did).