The losses of data in a packet based network, usually due to congestion and consequent buffer overflow. In VoIP (Real Time communication) the packets lost are not sent again and should not be over 5%.
PBX (Private Branch eXchange)
Private Branch Exchange. A small, privately-owned version of the phone company's larger central switching office
PDD (Post Dial Delay)
Time between the last number of the called phone is pressed and the audio signal ring or the called party is heard by the caller.
PSTN (Public Switched Telephone Network)
Public Switched Telephone Network. The traditional, wired telephone network.