Cloud telephony is relevant in a business context and is also known as cloud calling, is a type of unified communications as a service (UCaaS) that offers voice communication services. 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. Cloud telephony services enable users to place calls directly from any computer or mobile device with an internet connection and free businesses from the burden of buying and storing stand-alone hardware, like PBX boxes. The setup sample as below;

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.