Please read this
http://www.mobiion.com/forum/index.php/topic,312.0.htmlSince you have used "No Answer" for voicemail, you have only one option "Busy" to use.
Make a rule to filter undesirable numbers, move it to the 1st position, set its Action1 to Hang up, add undesirable numbers to [Phone #].
Other numbers will not be affected by it, so they go through and ring your phone, if no answer, they go to your voicemail.