In general, you can connect the OBi PHONE port to an available "CO line" or "loop-start trunk" port of a PBX. It may also be useful to connect the OBi LINE port to an available analog "station", "extension", or "SLT" port of your PBX.
For a better answer, please post: Make and model of PBX? Number of lines connected? Will you be dropping one or more when you connect the OBi? Do you have "system phones" (lighted button for each line, dedicated buttons for hold, transfer, etc.), standard phones (dial 9 for outside line, dial codes to access PBX features, etc.), or a combination? Does your PBX have an auto-attendant? Voicemail?
Also, please describe any special functions you would like the new system to perform, e.g. allow a caller to your GV number to ring a specific room.
