
I'd be a bit concerned, that as the landline was disconnected they may have moved. I would be concerned as to whether they still had the dog, or had already perhaps bred her. It's a difficult situation to judge without knowing the type of person involved. In my homevetting with a rescue centre it once took me nearly a year to track down a GSD which had been rehomed to someone (in which I had a personal interest as I had rescued her from a bad home and taken her into our rescue). When I finally tracked her down she had gone through 3 owners and when I found her had just been rehomed by the RSPCA in Gloucestershire! They very kindly asked the couple that had taken her if they would mind me contacting them, which they didn't and I get a Christmas card and photo's every year now! It was so hard to track her though. The man who had her originally kept telling me (when I phoned that she was fine, OK etc etc) He had moved and I found that out by going to his address and when it was obvious the house was empty, by questioning the neighbours (carefully and without giving anything away).
So, I think if it were me I would visit to make sure they were still there, and still had the dog. I think at that point I would play it by ear as to whether I took the dog back there and then. HOWEVER It could just be that they have got so attached to her that they don't want to part with her now, and that's perhaps something that all sides should have foreseen at the outset, especially if the two dogs have become really firm friends. It could then be quite cruel to split them up! and possibly quite traumatic for all concerned!
I do hope that this all resolves itself without too much heartache all round. Let us know what happens
Kat