how to manage phone no. format

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ASP.NET

Dear all,

There is textbox control in my webform for phone no(US format).

Here I dont want allow to enter the no in it except the

123-456-7890 format

My requirement is

When the user enter first 3digints into the textbox automatically the ' - ' should be present in textbox & again after entering 3digits the ' - ' should be present and aganin after entering remaining 4digits in it. after that i wont be allow to accept other no.s.. The same process should be run when the user erases the phone no into that field..

Thanks