I would strongly suggest, and not just for these shops, although they certainly qualify, that anyone doing mystery shopping register for a free online phone message mailbox. It takes a few minutes to sign up, and it's one more number to remember, but it's worth it.
You can set up your outgoing message and then when you get a phone call, your voice mail picks it up, if they leave a message, you get an email, letting you know that you have a message.
That way, you can give someone your "real" contact information, without dealing with the annoying phone calls.
So as to not promote any particular provider, simply google "Free online voice mail" There's a ton of them out there.