Your device's signal attenuation might be affected by a variety of things. Your position in a building, the weather, the existence of a given appliance, etc. It's impossible to go over each one individually. There are a plethora of possibilities that could have a significant impact on your outcome.
This is why the manufacturer has  designed and tested the high-quality GoDark signal blocking bags in a third-party engineering lab to guarantee that they function as intended under these variables.
You can put your cell phone in a dark bag for a good test and see how it performs. Get another phone and try calling the phone inside the bag as soon as it's closed. Instead, use a messaging app like Messenger or Snapchat on your tablet to send yourself a message. If the call or notice does not reach the remote device, the bag is most likely doing its job.