In a normal window? Then I'd close it immediately. OK, so they'd make it so that closes my call... Then I'd just bring some other window on top of it, and close it immediately *after* my call (if it didn't go away by itself).
Or do you think they would put their ads in one of those annoying "I'm-in-the-foreground-and-you-can't-put-anything-on-top-of-me,-nyah-nyaah!" windows? Do you really think so...?
Then how the FUCK would anybody ever be able to talk about what's on their screen?!? (i.e, what WAS there, before the ad popped up all over it.) No fucking WAY anybody with two brain cells to rub together would buy that!
(OTGH, that probably still leaves enough of a clientele to make it all worth their while... ;-/ )