Validate a Non-Required Phone Number Using Javascript

I have a javascript function that is supposed to validate Required Phone Numbers, checking for length and numbers/characters, etc...  I need to modify this to validate a Non-Required Phone Number (if the field is blank, do nothing;  if a user enters a phone number, THEN validate it correctly).   I've tried different variations, but don't understand javascript well enough to make it work.   Here's the original script:

function validateForm(f){
      if(f.userPhone1.value == ""){
            alert("Please enter the Presenter's full phone number (xxx-xxx-xxxx).");
            f.userPhone1.focus();
            return false;
        } else if (!validateUSPhone(f.userPhone1.value)) {
            alert("Please enter the Presenter's phone number in format xxx-xxx-xxxx.");
            f.userPhone1.focus();
            return false;      
      }
      
      if (CheckPhoneNumber(f.userPhone1.value)) {
            alert("Please enter the Presenter's phone number in format xxx-xxx-xxxx.");
            f.userPhone1.focus();
            return false;      
      }
      f.submit();
}

function CheckPhoneNumber(TheNumber) {
      var valid = 1
      var GoodChars = "0123456789-"
      var i = 0
      if (TheNumber=="") {
      // Return false if number is empty
      return false;
            }
      for (i =0; i <= TheNumber.length -1; i++) {
            if (GoodChars.indexOf(TheNumber.charAt(i)) == -1) {
            // Note: Remove the comments from the following line to see this
            // for loop in action.
            // alert(TheNumber.charAt(i) + " is no good.")
            return false;
            } // End if statement
            if (TheNumber.charAt(i) == "-" && i != 3 && i != 7) {
            return false;
            } // End if statement
      } // End for loop
      return true;
}

function validateUSPhone(str){
   str = strip("*() -./_\n\r\t\\",str);
   if(str.length == 10)
        return true;
   else
        return false;
}

Any help is very much appreciated!!!   I will award full points, as I need to solve this quickly!    Thank you!