I enter the phone_away data completely manually! However, if you are interested in automating a similar goal, you may be interested in Put phone away box .
If I had a bit less trust in myself and wanted an intermediate level of automation, I would make a spreadsheet and enter a “1” in every hour I didn’t use the phone, since I already have some scripts set up for sending data from spreadsheets to beeminder. Actually, maybe this is still worth considering.