Aluminum Foil Wrapping
So if you have a aluminum foil in your kitchen the it comes handy by making you unreachable on your cell phone.
Faraday Bag or Cell Phone Signal Blocker Pouch
remember to remove it from the pouch else you might miss important calls, so bottom line only way to now go out of network is just to enclose your device with metal it can be Foil, pouch with metallic enclosure or metallic box all works on the principle of faraday shield.
But be warned some times even enclosure with above objects also can have a slight signal leak and you might get the call and also above method is a battery drainer since your cell phone actively be searching for the network last but not least you might miss important calls so better use it as worst case scenario and try to attend all the calls.