VB Net Web Page input mask

I`m working on a small internal web site using Visual Studio 2012, with VB web form page. Our users would like an input box mask for phone number entry. Drawing a complete blank, can someone help with Ajax extender or another idea to write a simple phone number input mask in VB?

This is what I have tried.

1.      Added Scriptmanager to top off page
2.      Dragged and dropped MaskedEditorExtender into textbox
3.      Set Mask and Mask Type in properties box

However, I continue to get this error on page load:

Unhandled exception at line 67, column 5 in http://localhost:60760/WebForm1.aspx

0x800a138f - Microsoft JScript runtime error: Unable to get value of the property 'UI': object is null or undefined

And this line is highlighted yellow:

    $create(Sys.Extended.UI.MaskedEditBehavior, {"Century":2000,"ClientStateFieldID":"TextBox1_MaskedEditExtender_ClientState","CultureAMPMPlaceholder":"AM;PM","CultureCurrencySymbolPlaceholder":"$","CultureDateFormat":"MDY","CultureDatePlaceholder":"/","CultureDecimalPlaceholder":".","CultureName":"en-US","CultureThousandsPlaceholder":",","CultureTimePlaceholder":":","Mask":"999-999-9999","MaskType":2,"id":"TextBox1_MaskedEditExtender"}, null, null, $get("TextBox1"));