I know that, I was talking about the logistics. This was an employer who seemingly caught a conversation that the employer couldn't have known was going to take place. You have to know that something is going to happen or is likely to happen in order to get permission from the non-employee party. Otherwise, who do you ask for permission? Everybody your employees know or might speak to on their personal cell phones? Plus the non-employee party has to have some motivation for giving that permission. Then, it's highly unlikely that they are going to find any practical method for catching both sides of the conversation.
It's easy when the conversations happen on company phones because there is recording capability written into most employee phone systems plus most employee phone systems warn callers (and staff) that they are being taped.