jQuery validation, waiting for service call response before proceeding

I currently have a jQuery method that validates a UK phone number. This only validates the format and does not make sure the number is real, so I am trialing a telephone validation service that will.

The code below shows the jQuery validation method with the final IsValid(phone_number, 'GB') bit calling a function which deals with the service.

jQuery.validator.addMethod('phonesUK', function(phone_number, element) {
phone_number = phone_number.replace(/\(|\)|\s+|-/g,'');
return this.optional(element) || phone_number.length > 8 &&
    phone_number.match(/^(?:(?:(?:00\s?|\+)44\s?|0)(?:1\d{8,9}|[23]\d{9}|8\d{7,9}|7(?:[45789]\d{8}|624\d{6})))$/)
    && IsValid(phone_number, 'GB');}, 'Please specify a valid UK phone number');

The code below handles calling the service and works fine on its own.

function IsValid(telephoneNumber, defaultCountry)

{ 
  var internationaltelephonevalidation = new data8.internationaltelephonevalidation();
  internationaltelephonevalidation.isvalid(
    telephoneNumber,
    defaultCountry,
    [
      new data8.option('UseMobileValidation', 'true'),
      new data8.option('UseLineValidation', 'true')
    ],
    showIsValidResult
  );
}

function showIsValidResult(result) {
  if (!result.Status.Success) {
    alert('Error: ' + result.Status.ErrorMessage);
  }
  else {
    if(result.Result.ValidationResult == "Invalid"){
      return false;
    }else{
      return true;
    }
  }
}

My problem is that the jQuery validator method returns false before the response from the service comes back.

I did try a set timeout in the jQuery method and set a variable before returning from the showIsValidResult() function, but the variable was always undefined.

Any ideas please?

Thanks