I had a problem with popping sounds on my calls. Only internal users heard the popping, other end did not. In /var/log/asterisk/full: [Nov 5 04:24:35] NOTICE chan_dahdi.c: PRI got event: HDLC Abort (6) on Primary D-channel of span 2
Rebooted server and problem went away. Server uptime was 277 days. Trixbox.
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