Phones stop working for lots of reasons other than "I've been fired". I'd still show up to work. It'd be pretty bad if your phone indeed had just stopped working, and rather going in to get it fixed, you just assumed you were fired and then stopped going into work, thus actually getting fired for not showing up.

Most people there would probably come into work between 8 and 9am. Even if the phones were deactivated before then, most might not even have noticed (those that did notice might have had a premonition, but what are you going to do about it on your commute?).

That is a major plot point in Margin Call, which is a dramatization of the initial housing bubble crash of 08: https://youtu.be/zn2Xf9hAFcE?t=461

BES doing something reliably? That was a good laugh, thanks.