Validation for 10 digit phone number in Salesforce

Sample Validation:

OR( 
NOT( ISNUMBER( InfallibleTech__Phone__c ) ), 
NOT( REGEX( InfallibleTech__Phone__c , "[0-9]{10}") ) 
)

This validation is for for 10 digit phone number. If you use special characters like (,), -, etc., then you cannot use the above validation formula.