I think it’s important to first decide why you want to cut down on your phone use, and then to identify the barriers that have been getting in the way of you putting it away.
Sometimes it takes extreme measures like locking your phone up somewhere for a certain amount of time, giving it to someone else, or even running the battery out (if you can’t take the battery itself out). You can also have someone else set the passcode for you and change it regularly. Ultimately you have to decide what’s important to you and what you’re willing to do to get it. You may have to put up barriers between you and your phone. Identify what keeps you from putting it down. Are you afraid to miss a text or a call? Are you looking for stimulation? Do you feel unsafe without it? Those are also good questions to sort through.
In the end it’s not a question of willpower but it does require some effort from us to affect the change we want. If you have a provider like a therapist this would be a great project to work on.