Dear All,
Again thanks to PitzKey!
I did analyze the situation further, and I did solve it (hopefully). The issue was that - unlike during the previous years with Deutsche Telekom’s Voip service - one now needs to add the following the sip settings:
progressinband=never
For those who are interested in understanding the issue and how to analyze it via packet capturing on the WAN side:
In my examples, the call is never completed, just ringing and than hanging up. In all scenarios, completing the call does work without any issues whatsoever. The problem is just, that the calling party may not hear a ringtone presumably after a recent modification on the side of the carrier.
From the called party side, the ringing of a call to FreePBX does look as follows, if progressinband=never is not set explicitly:
Then, the ring tone is delivered as an RTP stream just like the call itself. If the caller is using our incument carrier, that RTP stream does get dropped - unlike in the past years until a few days ago. The striking thing is that if the caller uses another carrier (or even Deutsche Telekom mobile) calling us through our Deutsche Telekom Voip line, the RTP stream will not be dropped, but delivered. I gather that modern sound logo ring tones also indicated that the ringtone is often delivered as an RTP stream. Then using a mobile phone instead of Telekom’s voip/landline, only difference is that the codec is then g711A instead of g722. However, this is not a codec issue: If I limit our Telekom trunk to that classic codec, the RTP stream does get dropped as well.
Ringing does work differently when not calling FreePBX but an AVM Fritzbox, the notorious (positive) market leading modem router in Germany if one disregards Deutsche Telekom’s speedport boxes:
In that case, there is no RTP stream required to deliver the ringtone to the calling party. That does work with callers from Deutsche Telekom’s network as well as from other networks.
That led me to search how to avoid the “early media” issue. So far - more insights may follow, of courese - the solutions seems to be “progressinband=never”. One reads that this should have been the default, but that does not seem to be true at least in asterisk 13.
Regards,
Michael Schefczyk