Again, not an official response but, I don't know of any plan to phones for annotations. To be clear, annotations work on mobile tablet devices, just not on phones. But, I assure you, you don't want that. I've tried to annotate a PDF using other software on a phone screen with my finger. The experience is... less than optimal... "horrid" might be more accurate. There's just not enough screen real estate to make it worth doing.