In this case, ¹Õè is said at the end of a question. For example, the telephone rings...
This is very informal and you say this when you're pretty sure that it's not your boss or your parents on the phone, but your friend. You must have Caller ID (the service that tells you what number is calling you).
To make it polite just put ¤Ð or ¤ÃÑº
after ã¤Ã. Going back to the previous example,
This can be quite strong, especially if you say it when raising your voice.
For example you could say it with an angry voice,
If you say it in a normal context it only means "What is this?".