[osmocom-analog] PBX sends PRACK during call progress which osmo-cc-sip-endpoint rejects

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Markus Stamm markus at stamm.nu
Mon Mar 29 21:34:30 UTC 2021

Dear all,

please disregard my fist message in this thread - I was being a 
fast-fingered Freddy.

What I wanted to post is this:

With the help of the great examples on the mailinglist, I am able to 
successfully make incoming and outgoing C-Net/PSTN calls via Sipgate, a 
German SIP provider.

As my PBX is SIP-capable (it is an Auerswald Compact 5000R), I wanted to 
attach osmo-cc-sip-endpoint to an extension of my PBX.

The registration is successful, but when I receive a call, C-Netz (or 
Zeitansage) registers and accepts the incoming call, but the PBX does 
not understand the call is connected and keeps ringing the extension.

Using SNGREP, I believe I found the cause.

Following a 183 Session Progress message from osmo-cc-sip-endpoint to 
the PBX, the PBX transmits PRACK, and osmo-cc-sip endpoint responds with 
a message 504 Method Not Allowed. The PBX ignores this message 504 and 
keeps ringing the extension until I disconnect the call.

Sipgate, by contrast, responds by sending ACK, not PRACK, and the call 
connects successfully.

I'm considering opening a ticket with Auerswald on this and would 
greatly appreciate your expert views. To my understanding, a 504 Method 
Not Allowed should prompt the PBX to fall back to ACK, or at least 
realise there is an issue, and yet it seems to ignore the Method Now 
Allowed response from osmo-cc-sip-endpoint. Does this appear correct, or 
might there be an issue with Sofia-SIP?

I'm attaching the capture file in case this were useful.

Thank you so much in advance for your insights.

BR, Markus
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