A conductive enclosure attenuates radio: cellular, WiFi, Bluetooth, NFC, and GPS reception. Signal bags and boxes apply the idea to phones, keys, and radios during transport or storage.
The isolation holds only while the device is enclosed. A phone logs and queues data while shielded and transmits the backlog once removed, and cheap bags attenuate unevenly across bands, so testing a specific bag against a specific device is the honest check.