In the US, the phone company provides a U interface (2 wire, line
encoding 2B1Q at 144 kb/s, RJ-11 or RJ-45 connector), while in the rest
of the world, the phone company provides an S interface (4 wire, short
distances, AMI line coding, RJ-45 connector). Most ISDN interfaces can
connect directly to a U interface, but some (such as the Sun ISDN card)
cannot. A terminal adapter (TA) also offers an analog phone interface.
They require a so-called NT-1: