Help setting up Toshiba IP phone

I am a newbie with Asterisk and PBX technology in general.
I am the IT for a corporate office with 5 workstations. They all use the same phone, and unfortunately that phone is the Toshiba IP-5022SD. I have found virtually no discussion of this phone online, and the only manual I can find is one for the entire IP-5000 series here.

I set the “Primary IPU” on the phone to the IP address of my Asterisk server, but the phone says “IPU not found”. The phone does have valid IP settings. Both the phone and the Asterisk server respond to pings from each other.

I have yet to configure in/out trunks. I just want my phone to connect to the Asterisk server before I move any further. Please help me make that happen.

Thanks

Where is it documented that these support an open protocol, like SIP or H.323?