// CoAP server endpoint URL
void callback_buzzer(coapPacket &packet, IPAddress ip, int port) {
  char p[packet.payloadlen + 1];
  memcpy(p, packet.payload, packet.payloadlen);
  p[packet.payloadlen] = 0;

  if (packet.code == COAP_PUT) {
    String msg(p);
    payload message;

    lastFrequency = msg;
    message.buzzerFrequency = lastFrequency.toInt();
    bool status = network.write(header, &message, sizeof(message));
  }

  coap.sendResponse(ip,port,(char *)lastFrequency.c_str());
}