I think your frustration makes sense, but maybe you're looking at this from the wrong angle. When we take away the phone and force concentration, often the problem isn't laziness or lack of willpower - it could be that he's genuinely not understanding the material. Third grade is when things start getting more abstract and not everyone keeps up at the same pace. It's worth sitting down with him properly, without rushing, and trying to figure out exactly where he's getting stuck. Ask questions like "where didn't you understand?" instead of just correcting the wrong answer.
About the phone and concentration, yeah take it away, but not as punishment - it's just because distraction is real and kids' brains are still developing. Like, do 20 minutes of focused homework, then give him 5 minutes to rest. Changing location helps too - the kitchen table instead of his room, for example. And if the material keeps being hard, talk to his teacher. Sometimes we think it's inattention when really the kid just needs an explanation done differently, you know?
The most important thing really is to ease up on the pressure around it. A kid stuck with stressful homework gets more anxious and concentrates even less. Try to make it lighter, like "let's figure this out together" instead of "why can't you do something so simple?"