Go to twit2tel.com .
Just sign-in to twit2tel where you’ll be asked to authorize your twitter account.
Once you are done Click on the “Create a free account” button
Then go to “settings” and provide your telephone that you want to linked up to your Twitter username and click update next to the number. .
Example: tweet : @twit2tel call
Check this tutorial ;
Twit2Tel Now Offers Free Calls to More Than 200 Countries in: North America, South America, Europe, Asia, Africa, and Oceania
Link : http://twit2tel.com
If you have Jajah account , then also you can make free calls to any twitterer , check this post