There are a lot of good communication apps such as WhatsApp Messenger or Viber. They all use your contacts and let you do over internet whatever you used to do with your simcard. The difference between Telegram and those famous apps is the security on this application.
Telegram is a bit simpler than other communication apps. You may not see too many different icons and emoticons in its interface but it’s fast and secure. If you think your information is private and sensitive then Telegram is for you.
However, the problem is that it’s not as popular as those famous communication apps and therefor you may not find all of the people in your contact list so I suggest using it alongside Viber or WhatApp. Also it doesn’t have video chat!