Buy a cheap metal lunch-box/tin whatever(the stainless ones with clasp on metal lids are the best but anything metal that will fit your phone will work).

Spray the inside of the box and the lid with plast-dip or something similar but leave a strip around the edge of both parts bare so that you have conductive contact between the lid and the box, you could also glue foam on the inside of both parts

The point is a electrically insulated pocket that you put your device into that is surrounded by a conductive shell, you now have a effective faraday cage that will stop any signal.

You do however need to turn your phone completely off before putting it in there as it will continually look for a network until the battery is drained if you don't.

Something like this or this will work great

They might even allow it as carry-on on a plane if you just pull out the tin and put your phone in it before putting it in the tray when going through the metal detector.

Putting it in a zip-lock bag and wrapping it with several layers of alu-foil will also do the trick but that looks a a lot more(conspicuously so) tin-foil hat than a rigid tin/metal-box...

Buy a cheap metal lunch-box/tin whatever(the stainless ones with clasp on metal lids are the best but anything metal that will fit your phone will work).

Spray the inside of the box and the lid with plast-dip or something similar but leave a strip around the edge of both parts bare so that you have conductive contact between the lid and the box, you could also glue foam on the inside of both parts

The point is a electrically insulated pocket that you put your device into that is surrounded by a conductive shell, you now have a effective faraday cage that will stop any signal.

You do however need to turn your phone completely off before putting it in there as it will continually look for a network until the battery is drained if you don't.

Something like this or this will work great

They might even allow it as carry-on on a plane if you just pull out the tin and put your phone in it before putting it in the tray when going through the metal detector.