NEC Dterm DTZ-8R-1 Owner's Manual page 61

Issue 1.0
When Establishing an RF Link
If the TALK or Speaker key is pressed but the handset cannot establish an RF
Link with the base station, the handset emits an error tone and displays the
message "Out of Range".

Talk/Speaker
After five seconds,
returns to searching.
While Talking
If the handset cannot receive a signal from the base station in about five seconds,
it goes to Standby mode and emits an error tone. In this case, the LCD displays
"Out of Range".
If the base cannot receive a signal from the handset in about five seconds, it goes
to Standby mode.
Owner's Manual 6-17