I noticed one time when I received a call from the Asterisk console:
dial_exec_full: Unable to create channel of type ‘SIP’ (cause 20 – Subscriber absent)
In the context of the dialplan, I make a call simultaneously to two phones:
exten => s,5,DIAL(SIP/204&SIP/203,19)
Sometimes one of the IP phones is turned off, which is why this error occurs, informing that there is no subscriber.
To solve it, you just need to turn on the IP phone.
You can see information about SIP in the Asterisk console:
asterisk -rvv sip show peers sip show peer NUMBER quit
If the client’s IP address is null and expire is -1, the SIP client is not online:
Expire: -1 Addr->IP: (null)