The device state of this queue member

My logs are showing the following warning:

[May  3 19:47:27] WARNING[15641] app_queue.c: The device state of this queue member, Local/[email protected]/n, is still 'Not in Use' when it probably should not be! Please check UPGRADE.txt for correct configuration settings.

Any ideas what it means?

I’ve got a queue which is set to “ringall” on 3 extensions. It works fine, apart from that warning.