A lot of people seem to think it was Mike, the PI/fixer guy. On orders from either Gus or Saul. But why would Gus sic the twins on Hank, then warn Hank at the last minute? He wants Hank and the twins to kill each other? I'm not sure.
I like the theory that whoever made the call that it came as a result of orders from Gus. The brothers were a definite threat to his interests and were a thorn in his side. Hank was a threat as well and if he and the brothers were to kill each other, it would take care of those threats quite nicely.
Yes, Gus certainly loathes the twins. But Hank's beating of Jesse was about to deep-six his career. Gus allowing Walt to bring Jesse into the meth factory-lab saves Hank's career, keeping Hank a threat. If you think about this long enough, your head starts to hurt.
(I must say, I sure hope Hank lives. He has become one of my favorite characters. Beneath the Three-Stooges-like bluster, he's actually a really nice guy, and one of the few characters on the show to have a firm moral center.)