This actually depends on the sip trunk provider and what they will respond with when a busy here sip message is sent to them from the PBX and only when extension settings> forwarding rules > Available status and under the section 'If I am busy or my phone is unregistered, forward calls to:' is set to 'End call' for the external calls. Additionally, the option under settings > general 'Play Busy prompt when the extension is busy' is unchecked.
You can also change the prompt from under the settings > system prompts, choose 'manage prompt set', copy the prompt set and name it, change to this new prompt set then locate the prompt named 'NUMBER_BUSY' highlighting it and pressing browse choose a wav file that plays a busy tone, to replace the current one with, then again with the option of the extension settings > forwarding rules > Available status 'If I am busy or my phone is unregistered, forward calls to:' is set to 'End call' for the external calls, but this time with the option Play Busy prompt checked, then when a call is received to the extension that is busy with the above configuration, the audio wav file chosen from the previous step will be played to the caller.
