List all your DIDs by going to DID Table view. If the DID goes to the PBX internal modem, then it tells you in the table. You can also export this to CSV. Be aware that the previous installer could also have a way through the voicemail system by setting up a transfer mailbox. I don't know what type of voicemail system you have. For Toshiba voicemail, it could be an IVP8, Stratagy ES, or a MAS.

Also, look for DISA in the table view. Unless users specifically need DISA access, I would remove it.