Well, technically you do not need a phone to buy the case. The main problem is that carrying around a phoneless case will greatly increase the chance of a visit by those nice men in the white uniforms. They will give you an interesting long (very long) sleeve jacket but I don't think that it has pockets.
Bag phone? Hmmm... just threw away an old cell we had. The kind about the size of a brick and you had to pull the little antenna thingy out for it to work. If you guys saw my phone, you would laugh your azzes off. Doesn't interest me. My wife had to explain Facebook ediquette to me last night. I'm officially old... [standby]... "Hey you @#$!ing kids! Get off my grass!"