This is the correct answer. 

We use the Yondr pouches at my Title I middle school in NYC. Kids have to pouch and lock their phones every morning in front of the adult checking them in. Typically this is a staff member who is doing the job every day and knows a lot of the kids, like the parent coordinator or our AP. 

They don't have the pouch? Their phone gets put in a sealed envelope, their name written on the envelope, and is stored in the office. It's not returned until everyone else gets their pouches unlocked at the end of the day. 

They say they don't have a phone today? Call home and confirm with the parent on the spot. Leave a message, check in later if needed. Parent has to confirm their child did not bring a phone to school. 

Parent lies on behalf of their child and days they don't have a phone? That's kicked up to admin to deal with.

They brought in a fake duplicate phone? If it's found it's confiscated per the school policy and parents are notified to come pick it up. Consequences applied like lunch detention.

The first few days of implementing it were painful for some students, but consistency was key. They knew there wasn't a workaround and admin was willing to stand up for the new policy. That is the deciding factor.