I've never received a call before-hand, but I have found that once a delay or cancellation is announced, I have an easy update text message show up on my phone with the rebooked flights. I had this happen on MCI-DEN-SFO, when MCI-DEN was canceled due to mechanical. GAs were amazed that I was already rebooked and ready to print new BPs, only two minutes after they sent all of the pax out to the check-in desks to be rebooked.