If you want to make sure no one can listen in on your calls, then install PrivateGSM on your phone.
This anti-wiretapping software allows you to make secure (encrypted) and standard (unencrypted) calls from your mobile phone to any other phone that has PrivateGSM installed on it as well. Thankfully, you only need to purchase one license though, and you're able to send invitations to install PrivateGSM to anyone you need to speak securely to.
PrivateGSM utilizes a CSD data line to make calls secure, and activation of this line needs to be requested to your phone operator (this can usually be done for free). Make sure you check that a CSD line is available to you before you purchase the PrivateGSM software.
Making secure calls using PrivateGSM is very easy. You just choose the 'Dial secure call' option then select a contact. Providing that person has the software running your call will be secured using an advanced encryption procedure. This involves the use of AES256 session key, encrypted with the Diffie Hellman elliptic curve method. Don't worry if this sounds like gobbledygook - you just need to know that this is a proven anti-wiretapping system.
PrivateGSM can be set to auto-run when your phone is switched on, and you can access the program at any time by hitting 801 on your keypad.
If you're paranoid about who might be listening to your calls, PrivateGSM is a reliable tool to put your mind at ease.