Unless there's a problem with your "cut & paste", you did not make the
change I proposed. Verified and working here:
exten => 8600,50,Set(CALLERID(num),2000)
exten => 8600,51,VoicemailMain(${CALLERID(num)}|s)
Notice how VoiceMailMain also uses ${CALLERID(num}, not ${CALLERIDNUM}