Spying on Mobile Communications Anchor link
Mobile phone networks were not originally designed to use technical means to protect subscribers' calls against eavesdropping. That meant that anybody with the right kind of radio receiver could listen in on the calls.
The situation is somewhat better today with encryption technologies have been added to mobile communications standards to try to prevent eavesdropping. But many of these technologies have been poorly designed (sometimes deliberately, due to government pressure not to use strong encryption). They have been unevenly deployed, so they might be available on one carrier but not another, or in one country but not another, and have sometimes been implemented incorrectly. For example, in some countries carriers do not enable encryption at all, or they use obsolete technical standards. This means it is often still possible for someone with the right kind of radio receiver to intercept calls and text messages as they're transmitted over the air.
The safest practice is to assume that traditional calls and SMS text messages have not been secured against eavesdropping or recording. Even though the technical details vary significantly from place to place and system to system, the technical protections are often weak and can be bypassed in many situations.
See Communicating with Others to learn how to text and talk more securely.
The situation can be different when you are using secure communications apps to communicate (whether by voice or text), because these apps can apply encryption to protect your communications. This encryption can be stronger and can provide more meaningful protections. The level of protection that you get from using secure communications apps to communicate depends significantly on which apps you use and how they work. One important question is whether a communications app uses end-to-end encryption to protect your communications and whether there's any way for the app developer to undo or bypass the encryption.