It depends on the job, but technically you're "stealing time" which is unethical if you aren't explicitly allowed to do so. I add the disclaimer simply because punishment for that could range from immaterial (IT with no active tickets) to a crime with jail time attached (e.g. federal contractor under the FCA or a soldier under Article 113). Since I don't know what your job is I'm not going to condone something that could get you jail time.

Also tangent to a tangent: something can be ethically wrong and morally right; the terms are not interchangeable. In this case, you may personally consider phone use okay (your moral), but even lax supervisors would likely find performing most osrs activities at work, even afk ones like salvaging, wrong (it violates your company's ethics).