Hi Raj,
In the past when I have users that have run into this issue, for native functionality, I have typically made use of a formula field and a form rule to accomplish this for data entry on a form. For that to work I set up a field (In this example lets call it Phone Number Check) as a formula checkbox field. For the Phone Number Check field I used a formula of:
If(Length([Phone Number])=10, True, False)
What this means is this checkbox field only checks when the phone number is specifically 10 digits long. Then you can set up a form rule which checks to see if that Phone Number Check field is checked and if it is not checked then you can set it to Abort the Save and give them a warning message that they must use a phone number that is a valid length of 10 digits.
