@Candy:
She is now on her leash even when we take her into the yard as she would not willingly come back to us when we call her.
I adopted my currant boy from his breeder at 3 years old. I swear it was a good year before he really accepted that he was staying, and that his "mom" wasn't coming back for him. He had been socialized and had had a couple of hours free in her house daily, so not quite the same thing as your situation. But he was used to having lots of other dogs for company, even when crated, and being an only dog took some time to sort out. In your case, you have another dog for her to be with, but that can work against you as well as for you, since she isn't as reliant on you for company.
Perhaps do some work with her alone, if you aren't already, so she has to rely on you more. I would spend some time, perhaps with a clicker, and address the problem of her coming when called. Until you get that down, any unintentional release is going to be more problematic than usual with a Basenji!