Masked TextBox Control is used to for accepting or rejecting by validating it using required, optional, type
characters to be input. A very useful control to restrict user input as required.
Video:
Explanation for using Masked Textbox Control:
Using the Mask property the input type format is chosen.
Since for a telephone number appropriate mask is chosen.
The AllowPromptAsInput is set to true to overwrite the input as per the specified mask.