NEC SL1000 - Plug a Analog Telephone Directory Connect to the Outside(CO) Line Socket; Use the External Backup Battery Box(Option) to Connect each KSU

SL1000 Getting Started Guide
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2. Plug a Analog Telephone directory connect to the Outside(CO) Line socket
If your Outside(CO) lines are provided with a Analog telephone socket you can unplug the SL1000 connection
and plug a Analog telephone into the socket to make/receive calls.
You can plug a phone in each Outside(CO) line socket if you wish.
3. Use the External Backup Battery Box(Option) to connect each KSU
The external backup battery box (IP4WW-Battery Box) with batteries provides the power to the system
when the AC power is failed. The external backup battery box is connected to each KSU.
For More details, refer to Hardware Manual (Separate Issue).