There is an issue.

The big problem with Professional "Faraday" cages/boxes/bags - set aside how much they are efficient - is power supply, you need to provide power to the device in them.

A modern smartphone when isolated from network will start scanning all it can scan to try and re-gain a connection, the battery will go down much faster than in "idle" mode.

And battery duration in "normal" condition is already scarce enough.