In the old days when cordless phones used analog radio signals between the base and the handsets, those signals could be picked up by another device. However, today’s technology uses DECT (Digital Enhanced Cordless Telecommunications).
There are various versions of DECT for different countries that use alternate frequencies so as not interfere with other existing wireless (such as cellular, Bluetooth, Wi-Fi, baby monitors, etc.) in the respective countries.
DECT operates over ten different frequency channels in the 1880–1900 MHz band. Every call originated by any handset will automatically choose one of these channels, that is available, to communicate with the base unit.
Since the audio is converted to digital signals to communicate with the base, it is also transmitted in packets of data. This allows for multiple conversations to be shared within each of the ten channels by breaking it up into time slices.
DECT provides 24 timeslots per 10 ms frame, and each frame can be carried over any of 12 duplex channels—allowing a total of 120 conversations. The data is coded so that each base unit knows which data packets belong to its own transmissions. Therefore the chance of being able to piece together the conversation of another is highly unlikely.
In addition, handsets will only create a connection with its own assigned base by using a preset security code that must match. The technology, therefore, avoids a nearby user with a similar device from inadvertently eavesdropping on someone else’s phone conversation.