Skype is not a well crafted product:

- if you send a message to someone offline, and you log off, he will receive it only when both of you will be online and if you use the same device you used when you sent the message. If it was too long ago he might miss it. - if you use multiple devices with skype, you will have to read all everytime you change device

so even if you've heard of skype, I can get why there's so much hype for a well-crafted product like whatsapp, but for calls.