I couldn’t believe, how long it took me to find this simple answer, go ahead try googling “freepbx transfer a call to voicemail”, you will find forums with complex problems with transfers, voicemail and more.
Except my problem was, I simply didn’t know how to do it, so here it is.
1. if you are on the internal network and want to go directly to someone’s voice mail
press * followed by their extension ex: *2000
2. if you want to transfer an inbound call to someone’s voicemail
press ## followed by * and then their extension ex: ##*2000
if you omit the “*” it will ring that extension but by adding the “*” it goes directly to voicemail, quite useful, to leave yourself a message that get’s automatically sent to your email.