said by Glen1:Do you have an alarm system at home and on this line? Do you live in an apartment building where there is an entry system attached to the line?
No alarm, and no entry system attached to the line, Glen. I live in a duplex. (My neighbour doesn't have an alarm system either, BTW.) But I hear where you're coming from; these are reasonable hunches. Thanks for the input.
I knew this was gonna be a tough nut to crack...
Apart from the curious regularity with which this thing appears -- lately it's been happening exactly at 5:48 PM every time -- it's the interactivity that puzzles me the most. What sort of fault/tech issue can "react" to my taking the receiver off the hook? And the second "reaction" -- the reoccurence of the glitch on the same day -- is delayed by some 12-14 minutes. I tested this a bunch of times; I assure you this is not a mere coincidence. The quirk never happens twice on the same day unless I "catch it in the act" the first time around...
I should post a few more audio samples that include some extra sounds. Maybe those could give you, and others, a slightly better idea what's going on. I'm not optimistic, but I've got nothing to lose.
I'll be offline 'til the evening, but will respond to any replies then.