Nope. Not for everything. Perhaps every phone conversation, but I don't necessary use my smart phone for talking. If I, for example, work in 1Password which encrypts the data while synching, the NSA can listen in on that conversation and presuming they haven't broken my password or the companies algorithms, that conversation is not understandable.
If it goes into the modem encrypted, having the keys to the modem isn't going to help all that much.
And you're an idiot if you're doing anything remotely illegal on a cell phone system anyway.