Fix my phone numbers.js

// Oh thank goodness you're here! The last intern has completely ruined everything!

// All of our customer's phone numbers have been scrambled, and we need those phone numbers to annoy them with endless sales calls!

// The Format
// Phone numbers are stored as strings and comprise 11 digits, eg '02078834982' and must always start with a 0.

// However, something strange has happened and now all of the phone numbers contain lots of random characters, whitespace and some are not phone numbers at all!

// For example, '02078834982' has somehow become 'efRFS:)0207ERGQREG88349F82!' and there are lots more lines that we need to check.

// The Task
// Given a string, you must decide whether or not it contains a valid phone number. If it does, return the corrected phone number as a string ie. '02078834982' with no whitespace or special characters, else return "Not a phone number".

function isItANum(str) {
    let newstr= str.replace(/\D+/gi, '')
    if (newstr.startsWith('0') && (newstr.length ===11) ){
        return newstr;
    }else{
      return 'Not a phone number';
    }
  }