The IP Office Basic is set to DHCP by default on the LAN port.
However, it will will only pull an IP Address from your router if its LAN Port is plugged into your LAN and THEN the KSU is powered up.

There is no benefit to using its default IP Address.
If you did, you could not use the 3 free SIP Trunk Channels or Voice Mail to Email features.

You can go to extension 10 and use Feature 591 to see its current IP Address.

You can also press and hold down the Reset Button (with a paper clip) on the rear of the KSU and watch the CPU LED at the same time.
Once the CPU LED starts to FLASH, let go of the Reset Button.
It will take about 5 to 6 minutes to go back to Factory Default settings.

You said:
"The instruction sheet that comes with the IP Office box says that its default LAN IP address is 192.168.42.1"
"The instruction sheet also says the IP Office box will take an address from a DHCP server if one is available on the network."

192.168.42.1 IF there is no DHCP Server available.

PS:
The WAN Port is ALWAYS 192.168.43.1 and I have never seen a way to change it.