It is probably not worth debugging as Asterisk 13 is several years beyond end of life and chan_sip is no longer in the latest version, and has been, effectively, unsupported, for several years.
Generally we want the Asterisk full log, with, in your case, “sip set debug on” in effect, so see the full contents of the SIP exchange, not just a summary.
When you say no audio, are you just talking about ring back tone. I think the second sequence will stop Asterisk from generating any, as it is basically cancelling ring back.
thanks for your response
no audio means, that at the time the channels get bridged, neither the ring-tone of the called nor the voice is hearable when picked up (so no sound in both directions)