Well, as it turns out, it was the interrupt for the T1 card in the Asterisk. (Let me point out that it changed, apparently, due to multiple power hits, that were stopped by installing a UPS. Let this also be a lesson about the desirability of power backup!) Lowest priority, where it should have had very high priority. The problem description, upon further testing, is intermittent pops and clicks, even garbles that sounded like sync slip. 5 to 15 second repetition. Eventually any call would drop. 5 minutes, 20 minutes, whatever. And no significant help from Mitel trouble logs. Occasional sync loss, not good to see, but not directly related to every failure. Asterisk would occasionally show red alarm from Mitel, again not directly related to every failure. We 1st tested internal, ext to ext, no problem. Reconfirmed that problem was ongoing. Changed the 021 T1 card, no change. Changed out the interconnect cable, no change. Changed out the T1 card in the Asterisk and had to change drivers for different brand. That's when the low priority interrupt was discovered.
As for changing to a PRI, the card in place is the old 021 T1 only card. Not an option.
This may seem excessively wordy, but troubleshooting techniques seem to be needed by some of the folks that come here for help.
