Nspanel free GPIOs

Hello fellas, i want to connect a sim8xx-modul to my nspanel and so i need two free pins for uart. GPIO 19 & 22 are occupied with i2c bus in my setup. On the same connector there are GPIO 23 and 21; but that one is labeled with “BZ” for buzzer? Has anyone tried to use GPIO 21 for any external reason?
Any other ideas how to get a third uart out of the nspanel?

Cheers,
hellfish

Did you succeed on this? I was wondering how many there are free pins to use? I’m not able to find this information online.