I got it to work by declaring a variable first. For some reason it wouldn't work just using the substring method directly on the output:
var Phone1 = PhonePOC.rawValue
PhoneWithDashes = String(Phone1.substr(0,3)) + "-" + String(Phone1.substr(3,3)) + "-" + String(Phone1.substr(6,4))
Then I can use PhoneWithDashes elsewhere.