To hide a caller ID on a campaign, select the campaign you want to hide the caller ID on under "Voice Campaign" on the main navigation bar. If you have not yet created a campaign follow the campaign creation guide first to set one up.
Once you are on the campaign editor screen, select the "Caller ID" Drop down box and select "Hide caller ID." Do note you will still have to insert a caller ID number however where possible the number will be hidden. Due to carrier regulations, caller IDs will occasionally still be shown to consumers for network security.