DID
DTMF
DSS1

DID
Direct Inward Dialing; A specially configured phone line from the telephone company that allows for dialing inside a company directly without having to go through an attendant. A DID line cannot be used for outdial operation since there is no dialtone offered.

DTMF
Dual-Tone Multifrequency; Dual Tone Multi Frequency. The signal a telephone company receives when a telephone's touch keys are pressed. Also known as Touchtone. Each key has a different frecuency.

DSS1
Digital Subscriber Signalling System No.1 - a signalling protocol used between the ISDN terminal devices and the PBX.