You can setup an SMDR server in which this server will log any calls made by anybody. Software, like Tapit 2000 will allow you to keep better track of this information and can provide you with detailed reports of incoming and outgoing calls. The Tapit software runs you about $850, a special cable about $150 for the NEC unit is also needed. With a customer provided PC I think we charged someone locally $450 for installation and configuration.
I don't know if that is what you are looking for or if you only want to verify that specific tenants are using their specified outside lines, but let me know if this helps.