heh
I've always ran with the whole "name spoken three times in short succession draws attention" thing. What can I say, I like Beetlejuice and Lovecraft
There would be a 1%, 5%, or 10% chance of the being invoked taking some sort of action or more direct interest in what was going on. The percentage varied depending on how capricious the being is, and how involved they tend to get in the goings-on on the Prime. Of course, the chance is modified if they are on another plane or have some preexisting relationship to the being in question. And some beings (like Hastur or Beetlejuice

) always show up, no roll required.
Over the years Hastur, Bane, Shar, Aerdrie, Mystra, Lolth, Kiaransalee, Tyr, Beshaba, a few Chosen, Malkazid, and a couple others that slip my mind have been invoked in this manner. And almost all were the unintentional results of conversations had in game.
Sometimes players do the "godshot" thing, invoking a deity's name in the hopes of receiving aid. In that case, its similar to the above except I have the players roll percentiles. Then I roll, and if my result is within a given range of theirs something might happen.