A Lookup Source lets you specify a source for resolving numeric caller IDs of incoming calls to names. You can then link an inbound route to a specific CID source. You can also install the Phonebook module to have a small number to name association.
Once your lookup source is set u,p it can be used by "Inbound Routes" to modify the name field that displays on your phone's caller ID display.
Once your source is created, it can be used when creating or editing an inbound route.
See the User Guide