code: public void linkNumber(String msisdn, String country, String appId) throws IOException, NexmoClientException {
          UpdateNumberRequest request = new UpdateNumberRequest(msisdn, country);
          request.setVoiceCallbackType(UpdateNumberRequest.CallbackType.APP);
          request.setVoiceCallbackValue(appId);
          this.updateNumber(request);
      }
docs: Link a given Nexmo Virtual Number to a Nexmo Application with the given ID.
  
  @param msisdn  The Nexmo Virtual Number to be updated.
  @param country The country for the given msisdn.
  @param appId   The ID for the Nexmo Application to be associated with the number.
  @throws IOException          if an error occurs contacting the Nexmo API
  @throws NexmoClientException if an error is returned by the server.