Disclaimer: I am not a cell phone guy. I work on networks (primarily IP), but here's what I know.
Long story short, there's nothing you can do, personally, to secure your voice communications short of deploying your own network; even then, the phone-to-tower radio transmission is susceptible to interception. You can do a much better job securing non-voice traffic though. There are several issues, but to start: a) you are reliant on third parties (phone carrier's) for transport, b) you are reliant on a lot of old and/or legacy technology, and c) the phone's baseband (which actually handles phone-to-tower communication) is essentially a black box.
Encryption 101: Plaintext + Encryption = Ciphertext (encrypted text). The only 100% secure encryption is a one-time-password (OTP), where the message to be encrypted and the encryption key are the same length, and the key necessarily changes with each message; the OTP key also must be transmitted separate from the message and out-of-band. Every other encryption algorithm is reliant on the idea that it is mathematically unfeasible to decrypt the text in a reasonable period of time. The problem is computing power increasing, suddenly brute-forcing the decryption process isn't as unfeasible as it was 5 years ago. Furthermore, encryption algorithms that are thought to be secure are discovered to have breaks, which is essentially anything that can reduce the complexity of a brute-force attack. The point here is that encryption aren't a 100% guarantee your stuff won't be seen, and that there's a constantly moving target as encryption algorithms age. Also important to consider is that since technology only drops in price, the equipment needed to say, mimic a cell tower, eventually falls into the area where it's not unreasonable for a normal person to own. Technology needed to mimic a GSM cell tower became affordable over 5 years ago.
The next piece to the puzzle is that encryption/decryption takes TIME; as you might have guessed, the longer it takes for the encryption/decryption process, the greater the delay between when the data is sent and when it is usable, if you will, by the receiver. Voice communications are time sensitive, and even sub-second delays are noticeable to frustrating to users (there's been studies done to determine where that sweet spot is). Therefore, the most secure algorithms out there cannot reasonably be used to secure voice traffic. This is especially true when you consider that there's a limit to how powerful a processor you can fit inside a mobile phone.
Back to my original points. Because you are using a third party's network for transport, now you have to assume that your message can be intercepted at any point. If it's intercepted, it could eventually be decrypted (depending on the algorithm used, it might be near instantaneous, or it could be 1000 years). When you rely on legacy technology (i.e. GSM), you are also relying on the security techniques used at that time. Even worse, a lot of legacy technology was implemented with security as an afterthought, rather than built with security in mind. Since the phone's baseband is a black box, you are now relying on it to securely perform all of these operations. Transmitting voice over IP (VOIP) is a different discussion, but the user's options for security open up significantly.
Since non-voice traffic is not as time-sensitive, you can encrypt your data prior to transmission with the right tools. On a PC, there are tons of tools out there (even free ones) to securely encrypt files. I'm not sure how much is out there in the way of mobile phones, and that's why so-called cryptophones have a market to begin with.
This isn't intended to a "sky is falling" post. I just wanted to give a little insight into what security really means and how little control you have over securing your voice comms. Bottom line, everything you do on your cell phone or over the internet should be a calculated risk.
