How to limit a field ""Phone Number"" to only contain 10 digits?

Also, you may want to use a more elaborate check-box formula to handle entries containing non-digit characters and extraneous spaces. Something like the following, which handles up to five instances of a period, dash, parenthesis, or space entered as a separator between digits:

var text p=ToText(Trim([Phone Number]));
var text partOne=Part($p,1,".-)( ");
var text partTwo=Part($p,2,".-)( ");
var text partThree=Part($p,3,".-)( ");
var text partFour=Part($p,4,".-)( ");
var text partFive=Part($p,5,".-)( ");

Length($partOne & $partTwo & $partThree & $partFour & $partFive) = 10