That would explain why I never see it: I don't accept incoming calls, they tend to be useless, as it's either someone I don't want to talk to, or someone I've already hit 100 with. I can look into fixing it so that only one incoming call is allowed at a time, though.
The glitch seems to be mostly harmless, though it's annoying when they show up in the basement, as it messes up the APO management of visitor traffic flow. Since OFB and Seasons, my sims have more to do, so only Popularity or Romance sims have a ludicrous excess of friends. Hence my theory that it's the change in my own playing style which has lowered the incidence of this glitch.
What would happen to the other calls if only one incoming is allowed at a time? I typically ignore the 4pm calls since those are friends I
want to neglect if they've become so low, but some sims can use the contacts from outings to help fulfill their LTWs.