index.js

function getPrimePhoneNumbers(isoCountry, areaCode) {
  
  // ...
  
  // Get the length of the country code so we know what to cut off from the number
  let countryCodeLen = countries[isoCountry].code.length;
  
  // assign the resolution of phonePromise to a variable
  let primeNumberPromise = phonePromise.then(data => {
    var verifiedPrimes = [];

    data.forEach(twilioObject => {
      twilioObject.available_phone_numbers.forEach(e => {       
        if (primality(e.phone_number.slice(countryCodeLen)) 
            && !verifiedPrimes.includes(e.phone_number.slice(countryCodeLen))) {
          // Slice off the country code using the length calculated above
          verifiedPrimes.push(e.phone_number.slice(countryCodeLen));
        }
      })
    });
    
    return verifiedPrimes;
  });
  
  return primeNumberPromise;
}