Ericsson ZAT 19.2 Operation Manual page 271

14.1. Introduction
This modem is equipped with three types of test functions according
to the CCITT standard:
1. Local analogue loopback (CCITT loop 3)
Local analogue loopback (CCITT loop 3) with test pattern.
2. Remote digital loopback (CCITT remote loop 2)
Remote digital loopback (CCITT remote loop 2) with test
pattern.
(Not in CCITT V.21 mode)
3. Digital loopback (CCITT loop 2).
To execute the above test functions, the modem must be in the
command state.
Before test functions 2 and 3 can be executed, a connection must be
established with the remote modem. The modem must change from
command mode to data mode.
To return to command mode in order to execute a test, enter + + +.
"Activating the remote digital loopback ( with or without
selftest) during the CCITT V.21 mode is not possible.
PWR (Test) diodes on the modem flashes when test
mode is active. Local analogue loopback is not supported
for V.32 terbo.
You may also use the TEST switch, see chapter 5.