For all incoming calls, a queue can be set up with a cap on the number of members in the queue. The limit can be either 3, 5, or 10, and can be set up under Admin -> Phone -> Edit -> Settings.
After the queue message is played, the non-availability message will then be played, which will ultimately lead to the voicemail, if configured.
The maximum wait time in the queue would be 15 minutes, which is fixed.