If you put any extraneous metal in close proximity to your phone it will have an adverse impact on radio performance. Reduced performance will require the phone to increase transmit power to connect to the mast so defeating any nebulous claims of shielding from non ionising radiation.

People live in close proximity to high power 50kW+ TV transmitters and those at a range of many miles will exceed the field strength of a local 5W phone transmitter. After comprehensive studies by the UN World Health Organisation no adverse affects have been found in these populations.

Anyway radiation from space in the form of neutrinos is rated at a neutrino density of approximately 100 per cubic centimetre at any given moment and we are all still here

Richard