On the portage net-im/viber-13.3.1.22:0 On the developer site net-im/viber-16.1.0.37:0 Thank you. Reproducible: Always
I can't find this package having ever existed in Gentoo, but that version does seem to be in the 'rion' and 'calculate' overlays. 16.1.0.37 seems to be in another overlay, 'nest'. Maybe you need to switch overlays?
Yes, it is on Calculate overlays. How to switch overlays?
It depends on how you installed the overlay, but it would be best to seek support either on IRC or the forums.