Based off of https://www.briandunning.com/cf/391. Added comments to help a developer understand recursive functions.

phone-format

Let(

	[
		NumbersOnly = Filter ( Phone; "0123456789" ); // make sure we are only dealing with numerals
		NewNumber = Right ( NumbersOnly; Length ( NumbersOnly ) - 1 ); // trim the string by 1 for the next recursive call
		NewFormat = Right ( Format; Length ( Format ) - 1 ) // trim the string by 1 for the next recursive call
	];

	Case (

	not IsEmpty ( Format ); // This allows the function to exit.

		Case (
			// if the first character is a pound/hash
			Left ( Format; 1 ) = "#"; Left ( NumbersOnly; 1 ) & PhoneFormat ( NewNumber; NewFormat );
			
			// else process normally
			Left ( Format; 1 ) & PhoneFormat ( NumbersOnly; NewFormat )
		) // end case

	)

)