Autodialer - failed calls

Hello,
I have many failed calls when autodialer. Some of them are marked as unanswered, but most of them are marked as failed. If I dial theses failed numbers manually, I hear ringing.

In the latter case in my log I see:
[Oct 6 20:59:31] NOTICE[8606] pbx_spool.c: Call failed to go through, reason (3) Remote end Ringing
[Oct 6 20:59:31] NOTICE[8606] pbx_spool.c: Queued call to PJSIP/XXXXXXXXX@XXXXXX expired without completion after 0 attempts.

In case with calls marked unanswered, I have the following entries:
[Oct 6 21:14:42] NOTICE[9629] pbx_spool.c: Call failed to go through, reason (0) Call Failure (not BUSY, and not NO_ANSWER, maybe Circuit busy or
down?)
[Oct 6 21:14:42] NOTICE[9629] pbx_spool.c: Queued call to PJSIP/XXXXXXXXXXXX@XXXXX expired without completion after 0 attempts

In what cases may I get the result “Call Failure (not BUSY, and not NO_ANSWER, maybe Circuit busy or down)”?

Which end is sourcing ring back. If the far end is doing so, as early media, Asterisk will have the call in an progress, not ringing state.

May that happen, if the remote part instead of ringing plays music on hold?

If the remote end sends music on hold, it either has to use early media, or actually to answer the call. With most PSTN providers, it would have to actually answer, as they don’t allow early media, for commercial reasons.

We have traced both types of calls with the PSTN engineer. Asterisk ends calls with NOANSWER, when the remote part reports “no user responding”. With the latter case, marked with error in Asterisk CDR’s, in PSTN CDR’s calls are interrupted by Asterisk (side A). This is confusing, since they also must be marked with NOANSWER, not ERROR.