Your friend has Viber installed on his mobile phone and you have him on your contact list. But Viber still does not recognize him as a Viber user so you cannot establish a free call or send him a free text message. Well, you’ll have to count on a scenario like this one because it is possible in practice. The most probable cause is a different number format in your address book and the way your friend activated the Viber account.
If you have this problem, find a person who undoubtedly has Viber on the “All” contact list. Enter their details, change the digit, save and exit. Then enter their contact again, change the digit back to the original, the ”correct” number, save and exit. Wait for about 10 minutes and see if the person is now recognized as a Viber user.
If this doesn’t work, please try dialing the number manually in its FULL format (including the “+” sign, the country code, area code, etc.) through the Viber DialPad.
Actually, this problem has been known for decades. It used to ocurr already on the old mobile phones (your friend is calling and the phone does not recognize him as your contact). The best practice to avoid it is to have your contacts registered with full number format (+country code number).