For only checking the length of the number you could use:
if(len(PhoneNumber) <> 10, 'invalid', valid') as Check
But I doubt that this will be enough - normally you have to check for certain pattern from dialing code and phone number, various delimiter chars and so on. To check such things only with string-functions could be difficult. Therefore is it common to use regular expressions which unfortunately aren't be nativ implemented within qlikview - but there are workarounds: How to use regular expressions.