I don't think we'll have any love confessions any time soon... at least, not ones that go across, you know? Because like someone mentioned previously, even if Reino/Ren/Sho were to confess to Kyoko, she'd twist it into some weird thing and accept it as that.
It's still too early for some good romance. IMO. ^_^
I believe that the phone call was Sho's. It couldn't be Reino because he appeared too soon after that and never once mentioned about the phone call. I think if it was him, he'd say something. He's not a very secretive character. Not when things as simple as phone calls are concerned. It deffinitely wasn't Ren because he's too much of a gentleman to 'prank' call someone.
So, unless we're to see some new character soon, I think it was Sho. And that makes me think that he'll appear in the manga soon. Which depresses me because I hate any shoxKoyo moments. Even if it is completely insignificant. But, if that makes Ren jealous and brings out his Kyoko-loving side more, then I'm all for it. ^_^