Any wireless medium is essentially a radio station broadcasting the conversation. The range can be a few blocks in the case of a cordless phone, to dozens of kilometers in the case of cellular 400 and 800 phones. Scanners capable of monitoring wireless conversations are readily available from electronic stores such as Radio Shack. While these scanners are unable to determine the originating number, there are generally enough clues in the context of the conversation to identify at least one of the participants. More sophisticated cellular monitoring equipment is available that will literally allow an eavesdropper to enter a cellular telephone number in a software program, and capture only those calls into or out of that specific cell phone.
Fax machines are a further prime source of information leaks.
They are often the target of commercial espionage, since they are a concentrated source of important information that must be conveyed quickly. They use ordinary telephone lines and are easily wiretapped. It is completely unsecured. Transmitting email without the use of encryption is the equivalent of publishing it in your daily newspaper. The message can take any number of paths between you and the recipient, and can be monitored at any point along the way, whether for gain or just plain curiosity.