Voicemail only for legacy PBX

I have a Siemens Hicom 300 connected via T1 to a Freepbx server ver with Digium TE121BF card. Cards green on both ends but no send receive data in dahdi monitor when extension is dialed from legacy pbx to Freepbx. I’m sure there is some trunk/extension setup to be done but not a clue where to start. Would appreciate a sequence of items to complete to get the Freepbx system from where it currently is to becoming a standalone voicemail system for the Hicom. Thanks!