Not sure there is a correct strategy but you do want to avoid device & user, it is not well supported by FreePBX. What I’ve done that works, is to choose one extension to be the main extension set to the main extension number. All other supplemental extensions are added to the follow-me of the main extension with failover to main extension’s voicemail. Set the CID Num Alias for the supplemental extensions to match the main extension so that all calls appear to come from the same phone number. User must be instructed not to disable follow-me.
If you want to set hints so that the main extension appears to be busy when any extension is in use, use the technique here: