I just watched the film last night and was confused about the ending, too. I think there are several possibilities, based on the news lady talking about the "anonymous tip" and the kid's smirk at the end:
1. The dad was the anonymous caller and set up Perro for unknown reasons. Perro seemed like a loyal underling, so I don't know why the dad would want him arrested. Plus, if the dad did truly have a problem with Perro, couldn't he have Perro killed by another underling?
2. The dad was arrested with Perro but used his influence to get out of jail.
3. The dad was arrested and jailed but just wasn't shown on TV. Also, the mom saying, "I'm just glad _you're_ OK" (emphasis mine) made me wonder why she didn't say "I'm just glad you both are OK". The fact that the mom didn't say anything about the dad made me wonder...maybe the dad is truly gone. Is she happy about it?
4. (My favorite possibility) The kid was the anonymous caller, meant to setup everyone, including his own father. I'm not sure how this actually could have happened since he didn't have his cell phone, and he and Greñas only scraped together 5 pesos for the pay phone. Only towards the end do we learn that the kid knows that his dad wields extreme power and seems to have more confidence than a sheltered college student would have that they'd get out of La Bomba's alive. (Didn't he say to Greñas that his dad is known to get violent when angered? He may know if his dad's past). The kid's smirk at the very end made me feel like he wanted to inherit his father's money/empire. Perhaps he wanted to purge the ranks and start over, and he was pulling the strings all along. I also think he was faking being scared to pull the trigger, and he also didn't seem to really care that his friend Greñas was executed.
No matter what the writers intended, it's a great film.
