If you read the tech specs of IP phones, they will say the phone is “Full Duplex?” What does that mean?

Duplex, in telecommunications, means that data can be exchanged in two directions at once. A telephone, of course, is the most obvious example of a full-duplex device, because you can talk and listen at the same time.

Full Duplex is in contrast to other methods of transmitting data. A device that is half-duplex can send or receive data, but not at the same time, such as a walkie-talkie. In a simplex communication system, one device transmits, and the other can only listen, such as a baby monitor or FM radio. Multiplexing is when several devices share the same transmission line, such as an analog trunk.

Full duplex audio systems, because they both send and receive data, have the risk of feedback or echo. Echo cancellation hardware and software have been developed alongside the advancement of telephony for just this reason.

Additional Reading
What is Echo Cancellation?
Top Five IP Phones
– The Biggest Causes Behind Echo in VoIP