i have a server control text box for entering phone number. and I want to format the phone number in a way (111) 222 - 3333 (user input is 1112223333) as soon as user leaves the focus from the text box without post back.

Any suggestion or code snippet would be appreciated

I belive you would need to use javascript. Or a 3rd party control such as Infragistics.

You can use
WebInput from http://www.intersoftpt.com
or
MaskedTextBox from http//:asplib.net