Hi @MattForrest!
We have a PHP script which is setup to run as preflow on the account, invoked by Pivot (made in Callflows app).
This script can for intance handle presences (connected to calendar services, with a voice saying that a user is in a meeting and when he/she will be back, etc).
I guess it would not be that hard to add functionality to check if any of the user's devices is in use and then disconnect the call with a busy tone / response.
More info here:
https://docs.2600hz.com/dev/applications/callflow/doc/preflow/
https://docs.2600hz.com/dev/applications/pivot/doc/
https://docs.2600hz.com/dev/applications/pivot/doc/kazoo/hangups/