Spoof or blocked ID, are good for calling airlines or so where you don't want your number to show up, or be connected with someone else.
If it needs to be foolproof, ie, you are dealing with thing that are rather shady, either use a payphone, or cal shop (not around your corner), or buy a cheap cellphone with a simcard in prepay, and dispose at the same time of the phone and simcard (ie, don't use the same phone with another simcard, or simcard with other phone), and tuen off and keep off the cell as soon as you end your conversation.