[eli]

No, not end-to-end, but they are encrypted between your phone and the cell tower.

It's considerably more secure than an open wifi network.

[sp332]

GSM supports optional encryption, but there's no notification to the user whether a given call is encrypted.

[eli]

Yes, but I'm pretty sure all US carriers use it all the time

(Though it's true that this opens you up to a potential MITM attack by a well equipped adversary, it's still a far cry from an open wifi network)