I have an Asterisk 11 server connected to my SIP provider who is running a 3CX server. This is my so-called SIP trunk.
I can dial multiple outgoing calls through the trunk, but I can only receive calls from the trunk if it is not already in use - i.e. I cannot receive multiple inbound calls. If two people tries to call me simultaneously, one of them will succeed and the other will be sent to my SIP provider’s voicemail.
My SIP provider says they have set everything up to make multiple calls possible, and according to their logs it looks like as if my system refuses the second call (they say, I haven’t inspected them myself). However, according to my own tcpdump/Wireshark logging, they never sends me any INVITE from the second caller.
As I understand the article on https://support.it-communicationsltd.co.uk/hc/en-us/articles/203266111--How-to-Receive-Multiple-Calls-from-a-Ring-Group-Queue-or-External-Source, the 3CX server uses Phone Status while determining if it should try to call (INVITE) an extension or not. If the extension’s phone status is busy, it will not even try calling it (which sounds reasonable).
However, searching the web around, I cannot find much about the “phone status” flagging mechanism, and especially I haven’t found how to set up Asterisk to send phone status messages that my SIP provider’s 3CX server understands.