Because of the way that queue agents are implemeted in freepbx, it would be a non-trivial change to add ‘call confirmation’ to static agents in a call queue. However - this is easy to achieve by using a follow-me (or ringgroup) that has call confirmation enabled.
Create a follow-me or ringgroup and in that group, put the external number to be called using a ringall with confirmation strategy. Now use that follow-me or ringgroup as the number for the static agent in the queue. That’s it - when that agent is tried, it will use the confirmation enabled call features and keep the external agents from answering the queue with their cell phone voicemail…