Below is our purpose:
When the extension 501 and 502 are in one call, at this time, 503 calls 501, because 501 is busy, we can make the other extension 504 ring.
The setting of asterisk is as belows:
In the sip .conf:
and in the extension.conf:
Here 501,502,503 and 504 are all registered to our IP phones VI2006, in most cases, when 503 calls 501, 504 will not ring, and just play dir-last, it seems that asterisk cannot recognise the busy status.
As usual, if the sip server needs to recognise the busy status, we should close the call waiting in the IP extension. And in our IP phone, at default, it has enabled the call waiting.
So after you disable the call waiting, it should be OK.
It just is the test, and if you have better idea about it, you can leave a message for us. Thanks.