Cloud telephony is relevant in a business context. The capital expenditure in setting up a telephony system (be it a VOIP based internet telephony system or a PRI/PSTN based PBX system) is high. Cloud telephony remove this expenditure by moving it to the cloud.
Skype is a peer to peer VOIP communication application typically used in calling computers from computers. General public cannot call a skype system from their phones. Both parties need to be on skype and on a computer. So, from a business context, skype is irrelevant to handle customer calls. Skype/viber/whatsapp etc is useful for personal purposes.