Text Box

Hi..
Am new to asp.net
I am using a Textbox which is a server Control
is there any possiblites that we can extend the property of the text box
for exmaple.
in case of getting phone no input from user
the text box should accept only nos.. if error comes it should be displayed in alertbox

i want to create Custom property for the text box which accepts only numbers,alphabets like that

can any one give idea to do this ...