Reaching the target is precisely the reason why these shops are avoided by many shoppers, including me. There are a different companies that do them, but generally you have just spun your wheels for nothing if you are unable to reach the target or, having reached them and set up an appointment, they are a no show. Some folks are bigger gamblers than others, but the risk of an unsuccessful shop compared to the reward seems stacked against shoppers.
Because the associates know they are shopped frequently, any attempt to contact a specific associate is going to be a giveaway that you are a shopper. Several shoppers have experienced situations where they set up an appointment with the target only to find another associate there when they arrive because the target 'needed to ____'.
If you are going to attempt these, use *67 on your phone before dialing the number to keep you off of caller ID. Ask someone else in the house to ask if the target is there (preferably different gender). If they are, do a handoff of the phone if you need to observe phone courtesy and responses. "Here, let me let you talk with my wife." Unless you are not allowed to do so, set up the appointment as a couple--that isn't used a lot, I gather--and show up as a single, "My husband couldn't make it, but I would like to see what you have available."
You are going to have to get creative about why you asked for a specific person. Pulling names out of thin air rarely works well. Try something like "My husband works over at _____ and one of the guys there told him his wife had talked to you." Of course ____ is any large employer in the area and you have three layers of separation: 1) your husband, 2) his friend (who your husband may have named but you 'just know him as Jason, never heard his last name', 3) the friend's wife (who you never heard a name for at all).
Take only jobs that are near enough to you that you could return 2 or 3 times easily to meet with the target in case they don't show up for an appointment.
The general complaint I have heard about with the companies that offer these is that they are very accessible when they are trying to get you to take the shop, very helpful over the phone before the shop and impossible to find if/when you have any kind of issue or problem. So ask your 'what if's before you take the job and make sure you have d direct phone number in case you have issues or problems later.