Personally I think the phone idea is excellent. It's a great way of incorporating an interface into a game without removing the player from said game. You can show missions (as text messages), you can show maps, you can let the player take photos, record audio/video, talk to NPC's, etc., all from one single gadget. You can have easter eggs (find a phone number in an obscure newspaper, call it for some unforeseen effects), prank calls, mini games (Snake!), whatever you want. I love the idea.