FreePBX currently uses "device" <devnum> for the callerid field of the device since macro-user-callerid configures the proper user displayname. Similarly, we default to mailbox=devicenum@device where 'device' is a reserved 'context' and then we symlink the actual mailbox directories. We do all this since adhoc devices can be logged in/out of and we need to make changes without regenerating and reloading config files.
For fixed devices, we can lookup the associated user and set the actual CNAM and mailbox context (or skip the mailbox= field all together if there is no voicemail for that user). This has the benefit of quicker response to MWI updates on fixed devices then waiting for the next Asterisk polling interval, and also in some cases easier to follow CDR records and CEL events because of the actual technology CNAM setting prior to being changed by the dialplan.
This is a feature request but does indirectly address some bugs and will thus be considered in 2.10+