There are several aspects to consider, with the first being the interface of how the phone system is made aware of the check in/out function for the guests. As Kev points out this is typically accomplished by a property management system (PMS) which should have a telephony interface/protocol. There are a many different PMS protocols out there and you need to find out which one is in use so you can understand the capabilities, the command set and the interface type - serial, TCP/IP or ?
The PBX system may or may not do call cost accounting. Those that support some form of standalone hotel operations (meaning no PMS) generally do have a rudimentary costing capability, but many PBX systems to include the UCM do not have internal pricing functionality. In this case the CDR or SMDR records are passed from the PBX to a 3rd party call accounting system that parses the data, determines local, long distance, international, and other call types and may or may not price the call, but will at least pass the info to the PMS which may price the call.
As far as generating a message for a display, you will need to see the PMS routines for check-in and then have a middle ware or other to see the check-in message that can then generate the API command to send a text, if such an API into the UCM exists (Never checked). Of course, doing that may also trigger the phone into a mode that allows the guests to reply.
So, find out the PMS and then check the PBX API to see if the text is supported.