Ericsson ZAT 19.2 Operation Manual page 89

"3. Command sets _
+&T = Inactivity timer control
Commands | +&T0, +&T1, +&T2, +&T3
AT+&T3 piel Inactivity timer used on both received and
transmitted data.
Selects the operation of the inactivity timer (+8).
Remember to assign this register with a value indicating the
period of time with no data that is allowed before disconnect.
In switched line mode the modem will disconnect the line at
inactivity timeout.
In teased line mode the modem will disconnect or initiate a dial
backup on switched line if inactivity timeout occurs.
Associated command +&DB.
+U Data carrier detection - extended functions
Commands : +U0, +U1, +U2
Enhanced control of V.24 circuit 109 (DCD)
AT+U0 gi] - DCD (109) OFF during tests where DTE is nol
Stages a active.
AT+U4 er -=atz-] DCD (109) ON during tests.
AT+U2pial DCD (109) always ON, except during
disconnect.