Depending on who the "snoop" is, phone calls can be just as insecure as snail mail or email. In today's world, privacy is a luxury, not a right.
Private key encryption, where you and the other person need to share the same password, is only as secure as the weakest link in the chain. Most private key encryptions don't use strong encryption methods anyway.
Public key encryption, on the other hand, has stood the tests of both time and government. It has the advantage that it doesn't need two parties to agree on a password.
The use of high-end public key encryption is so strong that not even governments can crack it (which is why government doesn't want you to use it).
If government doesn't worry you, then you can use Verizon or one of the several equivalents. Otherwise, you must use a private program.
PGP was mentioned. There are other open-source and free programs available, too. Look around download.com and sourceforge.net, among others.
By the way, PGP provides (at a cost) a method of encrypting phone calls via VOIP over the computer, so that you can chat securely to another person on the other side of the world. I know there are other companies offering similar facilities.
Paddy