Q: How to make a regular call from a wifi ipad using icloud account?

If you have an iPhone AND an iPad both running iOS 8.1 then you can make and receive calls on your iPad through the iPhone. Both devices must be on the same WiFi network...

http://www.redmondpie.com/how-to-use-ipad-to-make-receive-cellular-calls-in-ios- 8-via-iphone/

Just like Sooner Al said, you need to be on the same wi-fi network and signed into via your Apple ID. Have a look here for the official guidance:

Make a call over Wi-Fi with iOS 8