No trunk needed, add something like
exten => NNNN,1,playback(custom/yourmessage)
exten => NNNN,n,trysystem(rm /var/spool/asterisk/voicemail/default/8000/INBOX/*)
exten => NNNN,n,hangup()
to the [from-internal-custom] context in /etc/asterisk/extensions_custom.conf
where NNNN is the number to dial in a "misc destinations" of choice.