See Reference Point T.
"T interface." YourDictionary, n.d. Web. 16 August 2018. <http://www.yourdictionary.com/t-interface>.
T interface. (n.d.). Retrieved August 16th, 2018, from http://www.yourdictionary.com/t-interface
The interface used between network terminator 1 (NT1) and 2 (NT2) in an ISDN connection. This is an internal interface in the U.S., but external in other countries. See ISDN.