403 Maximum parallel calls exceeded

Hi guys.
I do not understand, why this is happening, please help…

In my test environment my ITSP makes 2 concurrent lines available for me. In a specific situation I get this 403 error mentioned in the subject and I do not understand why.
Here is, when this happens:
I have an active inbound call (number_caller) which is handled and hold via IVR. IVR has an option to forward the call via Dial() to another national Number (number_callee). When I choose this option and Asterisk is dialing, I get the messages below.

In my understanding, in this scenario I only used 2 concurrent lines. Am I wrong?

;;;;;   -- Called PJSIP/xxxxxxxx@itsp_endpoint
;;;;;  == Everyone is busy/congested at this time (1:0/0/1)
;;;;;    -- Auto fallthrough, channel 'PJSIP/itsp_endpoint-0000002f' status is 'CHANUNAVAIL'

<--- Received SIP response (626 bytes) from TLS:ip.my.itsp.tld:5061 --->
SIP/2.0 403 Maximum parallel calls exceeded
Via: SIP/2.0/TLS 192.168.x.y:5061;rport=44453;branch=z9hG4bKPj602317f7-cae4-4527-8919-05dab8431951;alias;received=my.public.ip.address
From: <sip:number_caller@sip.itsp.tld>;tag=a3cc46f0-a51e-4e3d-ba08-de6e067ad791
To: <sip:number_callee@sip.itsp.tld>;tag=f3067022b00c564156251ba2f28f331f-3321
Call-ID: 9ff93547-7cd1-40e6-bbd7-e0e207bf28f7
CSeq: 11446 INVITE
P-NGCP-Caller-Info: <sip:number_caller@sip.itsp.de>;ip=ip.my.itsp.tld;port=5060
P-NGCP-Callee-Info: <sip:number_callee@>;ip=;port=5060
Server: Sipwise NGCP Proxy 3.X
Content-Length: 0

You recieved the 403 from your provider…

Maybe ask them…?

We cant tell you how many line you are using… You only show part of the signaling.
I would start with your provider, they can tell you the exact reason you get the 403 message.

I like to know how asterisk handles these connections.
Is it possible that asterisk and its handling of connections will consume up to 3 lines in the scenario i was describing?


What’s a line?

For a simple call, Asterisk will only use two sessions.

However it is possible that the two sides will disagree as to exactly when a session has ended (actually they always will, to some extent).

Also you would get this result if there were two incoming calls, before the ITSP detected the outgoing call for the first to arrive.

Could you please guide me to some documentation to get a better understanding of what is going on in the context of a session/line, etc.?

“Line” means whatever your ITSP wants it to mean.

For session, see, for example, https://en.wikipedia.org/wiki/Session_Initiation_Protocol