SIM800H responding with BUSY


I am creating a gate system using a Raspberry Pi Zero and a SIM800H (Adafruit Fona). I have been working on this project for about 2 years and feel quite comfortable with the software stack. Everything is done in Nodejs. Here is what happens when I call the SIM800H:

  1. I call the SIM800H.
  2. SIM800H responds with URC’s RING and +CLIP. As expected.
  3. I send ATH to the SIM800H to hangup the call.
  4. SIM800H responds with BUSY after about 200ms. <= what is this?
  5. SIM800H responds with OK after about 9ms. This is normal. As expected.
  6. The call is ended. As expected.

Now, what is the BUSY response from the SIM800H? I cannot find anything related to BUSY in the SIM800H documentation and I would like to know what is the cause for it.

Thank you in advance for flexing your brain for me :slight_smile:


The ATH command is like hitting the decline button on your phone when you are getting a call. Its like rejecting the call. Even when you are ringing to a phone. If the other person disconnects, you will get the BUSY response in audio.

Hi Ravi,

Thank you for the reply. The SIM800H actually responds by writing the text “BUSY” to the serialport. That response is a bit strange since it is not mentioned in the documentation. According to the documentation after the ATH command is sent to the SIM800H the response should only be OK and not BUSY and then OK.


ATH is one of the commands defined in the V.25ter document (written by the ITU-T and not SIMCOM), so it’s functionality might be actually specified in that document.

Thanks for the pointers. I understand what the ATH command does. That the SIM800H responds with “Busy” after issuing the ATH command is still a mystery.