Can I use an iPad (Wi-Fi+Cellular) as a cellphone?
No, you can't use it as a carrier cellphone. But if you have enough data in your plan, you can use it as a cellphone through services such as Google Voice or Skype which can give you a phone number that you can call with and be called with over the internet. They both provide apps that will "ring" your iPad when someone calls the number.

Google Voice is free to have a phone number. Calls From the US or Canada to the US or Canada are free, and calls to or from other places are pretty cheap.

Skype charges for a phone number, but that includes some sort of fee that gives you a predetermined number of minutes.