A PBX (private branch exchange, or phone system), VM (voicemail), and CAS (call accounting system) must all be interfaced to the PMS (property management system).  Interfacing via IP is on the way, but is still done through serial connections across physical COM ports.

The PBX interface is responsible for putting the guest name on the phone for the purpose of caller ID, as well as locking/unlocking outbound access of guest room phones.  It is important to note that new Mitel systems allow designating 911 routes as "EMERGENCY," which allows locking down vacant rooms to internal calling only; older Mitel systems don't allow the emergency route designation and should leave vacant rooms with local calling access.  The emergency route designation overrides all other routing restrictions, except for a COR restriction blocking an extension from accessing an ARS prefix.

The VM interface is responsible for opening/closing VM boxes so that new guests won't get messages left for the previous guest.  I heard a rumor at some point that early software deleted messages on check-out, but Mitel got sued over this practice and messages are now retained until check-in of the next guest.

The CAS interface is responsible for pricing calls and posting those prices to the PMS so the calls show up in a guest's folio.  If a CAS posts a cost for a room that a PMS doesn't know about (admin phones, meeting rooms), or if it posts a cost for a checked-out room, the PMS will put the record in a house account that the controller will have to audit out.  For this reason most properties now prefer to monitor administrative call records via the nightly audit from the CAS, but to not allow administrative call records to post.