If / Else Expression Logic

The program i am working on is to take string from the user which should be a phone number then it will return true or false based upon the method that checks to see if it meets the criteria of a certain format if does not it will return false otherwise return true. I am working on a if/else structure that will print this number is valid if it meets the criteria of the method i previously mentioned or return the number is not valid if it is false. the logic of the expression i put in the parentheses of the if/else statement. This is what i have so far:

if(){
System.out.println("The phone number is valid");
}
else {
System.out.println("This isn't a valid phone number");
}

Do i need to compare it based upon the method that checks if it is true?