How to make phone not reachable without removing battery /SIM ?

Recently i was searching android application that can make phone not reachable, but surprisingly i did not find any, being android mobile application developer i thought of writing my own app and upload it to google play but i did not find any api's in android SDK(Software Development Kit) to do so. So i was searching for "Call Not Reachable" app like every one else, i was in need to avoid a caller but don't want him to know that i am not picking up his call intentionally and also i don't wanted to switch the phone off  which means to him i intentionally switched  off  the phone. But i just wanted him to hear "the subscriber you are trying to call is not reachable" or "Mobile number you are trying to call is out of network". and this would be a real excuse for me without hurting the caller. So i was baffled with the question is  there any way to go "Out of reach" on my mobile phone without removing the battery or SIM? though i googled extensively i did not find any application or tricks to do it, but suddenly BAM! i remembered testing team in my company using a trick to make phone not reachable to test our mobile  application for some scenario using ALUMINIUM FOIL.

Aluminum Foil Wrapping

Yes! the aluminium foil which is used to wrap food does the trick, wrapping your mobile device in aluminium foil blocks cell phone signals, this effect is called faradays cage or farady sheild an electric charge exists on the outside of a charged conductor, and the electric charge has no effect on anything enclosed by the conductor, Wrapping a cell phone in aluminum foil creates a Faraday cage. Because cell phone signals are electronic, the aluminum foil prevents the signal from reaching the cell phone.

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

Just search for signal blocker pouch in amazon you find tens of them and yeah it really works its easy to use, just put your mobile device in this pouch  and your are unreachable these mobile cost some where between 4$ to 20$ depending on make.
                                       
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.