Most phones will lose SOME signal...
After all they use near-microwave frequencies, and microwaves are rather infamously fickle when it comes to travelling through anything that contains water (like, oh, humans).
However, putting the bare metal of the antenna outside the phone is pretty much guaranteed to futz with the signal severely when anything conductive (like, oh, humans) touches it.
Worst part is, if you try and fix this by putting some kind of sticky plastic or lacquer over the bare metal, what's the reckoning you get rid of the conductance problem and start introducing a capacitance problem? Iphones leaking signal all over the spectrum? Whups!
Mine's the one with an old FM radio in the pocket, that strangely enough gets a better signal when you touch the antenna.