[asterisk-users] When should a Progress or Ringing be used in a today's telephony ?

[Olivier]

Hello,

Thinking back to my current practices, I would be very curious to share
here about when should applications such as Congestion, Progress or Ringing
be used in today's telephony.

I would define today's telephony with:
- SIP phones
- Asterisk
- a SIP trunk to an ITSP
- fixed or mobile lines reachable through this ITSP


1. When Asterisk receives a SIP call coming from PSTN, is there a time
frame within which Asterisk must reply something to keep caller from
canceling the call ? Where does this limit come from ? From SIP RFC ? From
local regulation bodies ?

2. Which SIP signal is required to stop call cancellation in the previous
case ?

3. When Asterisk receives a call, either from PSTN or from a SIP phone) it
cannot process (unkown callee, whatever reason, ...), should you stop
processing with Hangup or Congestion ?
Hangup application allow for exit code customization, if I'm not mistaken,
but  Congestion exists for a reason.

4. Is it a good practise to send a 180/183 when you don't get one ?

5. I observed I sometimes got  a 100 Trying then a 183 session Progress
when outcalling some (mobile) phones while simpy getting 100 Trying when
some other (mobile) phone through the same carrier (most probably, end
devices were not managed by the same (mobile) telephony provider).
What explains such difference ?

Best regards
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