This may be spoilerific, but then again, this is a mystery manga.
Nishina is suspicious, isn't he? For a while, I thought he was indeed the Prince, despite his denial. He's always with Teshimano, who was Vince and accompanied the Prince all the time. The only reason I've abandoned that theory was because the manga heavily implies (by that I mean almost tells us outright) that Zeze is the Prince.
So who's the voice behind the phone? I don't think it's anyone special. I think it's just Nanaura and Nishina discussing what to do. But why are Nanaura and Nishina trying to find the Prince? Nishina claims that he wants to be of help to him, but he's not actually there. The only other person from Moswick who's on their side is Ōki, and she wasn't aware that the "Prince" didn't exist. So I can't believe that Nishina is doing this out of loyalty when no one else from Moswick is looking for him.
Also, I think Ashley, the person Nishina is supposed to be may be the one Nanaura killed. Nanaura sees a vision of a Moswick soldier, and I'm guessing he's the one Nanaura killed. There might be some resemblance, but the face is obscured, so it's difficult to tell.
So if Nishina isn't Ashley, who is he then? My guess is now that he is Lucas. Lucas appeared to have some dealings with the Prince, so it makes sense that Nishina is looking for him. Nishina, despite being a knight, knew about the healing magic that only priests can use; even Minami thought healing magic didn't exist. The current theory is that Nishizono used healing magic on Bart, a non-priest, and lost her ability to use magic afterwards. So the next time Maki tries to use magic, we'll know if Nishina is a priest or not.