Trunking NEC DSX 40 system to Freepbx?

Has anyone tried trunking from an NEC DSX 40 system to an Asterisk system using FreePBX? I can’t seem to find any documentation and trying to get any help from NEC is like trying to pet Cerberus behind the ears.

Ar you trying to hook them together with SIP trunks, or something else?

I am trying to use SIP trunks.