Putting any case on a phone that interferes with the cell signal is only going to make a "non problem" worse. Let me explain. Say your 250m away from a cell tower and your device is in your pocket. Unless your wearing aluminium trousers the amount of radiation you absorb from the cell tower remains constant, in fact if you walk away from it the amount (which is tiny and insignificant) lessens until you wander into another cell towers range. However your phone as it loses signal will increase it's output ergo it's amount of radiation it puts out. This is why a poor cell signal reduces battery life.
Now stay 250m from the cell tower and put a case that blocks radiation on your phone. The phone is going to work overtime to try and keep a connection. Increasing it's output of radiation to it's maximum. You'd be defeating the object of reducing the amount of radiation absorbed by your body by putting a radiation inhibiting case on.
Don't take my word for it, it is basically simple physics. For what the US and EU regard as safe SAR (Specific Absorption Rate) levels and what individual devices put out and the levels cell towers broadcast at are all here with more information from links there:
What is SAR? Definition of SAR: Glossary Entry on Cell Phone Radiation