Today is 25th. It's night in Japan right now, so Mello must be getting ready for his plans. I've been meaning to write this small detail for a while, and I guess that it's the perfect moment now. But it has spoilers from manga chapter 99, so please avoid them if you don't know about the Death Note story.
In the manga, Near tells Halle to find Takada's whereabouts and stop Mello. The only way for that is, naturally, to contact Mello and ask him what's going on. Near told Light that he had the means to contact Mello, albeit he doubted that Mello would listen at that point. But now I'm inclined to think that Near might have been bluffing about Mello not listening.
This is because of the following. Takada called Light as soon as she killed Mello and told him where she was. Light and the police managed to locate the phone signal and headed to Nagano as soon as possible, being the only ones who knew about Takada's whereabouts. When they arrived, Halle seemed to be there already (this is even more clear in the anime, where she seems to arrive shortly before Light does).
So I guess that Mello did allow Halle to contact him for real, and he told her about his whereabouts or simply let her to get the phone signal. If Mello let the SPK know where he was, this only adds to the theory that he did it to support Near's plan, as a way to indirectly cooperate with him. If Mello had been doing it for himself and just for himself, I don't see why he would let Halle gather the information Near wants about his location; obviously, Halle would call him in Near's behalf, which is what she did.
What are your thoughts?