Universal Personal Telecommunication (UPT) - Access Devices Dual Tone Multi Frequency (DTMF) Sender for Acoustical Coupling to the Microphone of a Handset Telephone
Addendum 1 to ANSI/NCITS X3.100-1989, Interface Between DTE & DCE for Packet Mode Operation with Packet Switch Data Communications Networks - NUI and NUI-Derived Facility Extensions (formerly ANSI X3.100-1989(R1995) Add.1:1991)