Is Arch Linux Forums the right place to ask this question? Your question is not Arch specific, not even related to Linux. (e.g. pairing a phone is necessary in windows too)
As an answer for your question: You need a smartphone, for at least whatsapp. Computer clients for whatsapp rely on a smartphone, they are not actual recievers of messages. If the smartphone gets disconected (runs out of battery, loses wifi connection) the computer no longer recieves messages. I do not know Viber, though.