[Radii]

I thought the call was from gus, didn't think much of it:

Spoiler

UPDATE: There's been so much debate in the comments about who called Walt - Gus or Mike - that I went and asked Vince Gilligan. I don't view his answer as any kind of spoiler (as it's info from an episode that's already aired), but in case you do, don't read the next short paragraph:

So as it turns out, it wasn't Mike. And it wasn't Gus. It was, instead, the new third man in the operation, Tyrus (played by Ray Campbell), whom we saw earlier weighing the batch. And that explains why no one could agree on whether it was Esposito's voice or Banks's voice, I suppose.