Free call indicator for iPhone users

Mark Frauenfelder:
David Pogue passes along a good tip for iPhone users on his blog:

You know, of course, that in general, cellphone calls to other people who have the same carrier as you (like Verizon or AT&T;) are free, right?

But how do you KNOW when somebody with the same carrier calls you? How do you know it

• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
•