not casing at all! I have the phone form 2 weeks and I am pissed about this, I observed it with the factory rom, tried different roms after that still the same, I came to xiaomi form balckbarry with superb coverage and this is not acceptable for me.
the problem is when the 3g coverage is not ideal... then the phone start to change between - 2g for couple seconds, Emergency calls for couple minutes, No Service for couple minutes, 3g for couple seconds and again the same in different order! so thats means that most of the time there is no coverage! Even if I change the network setting to 2g only the phone still does the same think but without going to 3g... Some times if I make call the other site does not hear me but I always hear them.
OK what i tried that work a little bit...
Previously i used THICK casing with alot of plastic molded thing.[Actually my its my wife's phone].
So i really thoughts it the casing fault. So i decided to open up the casing. immediately the 3G line became available.
I compare side by side with my Mi2S phone for its Signal strength [ About>Status ] Look into Signal Strength ...
If reading going to upwards to near "0" means signal better.
Then wat i did is to put back the casing and HELL the line works wonder. The strength are back.
From here i assume that the cell tower trying to get "usual" registered phone and line became easily available.
I again test on my parents' house where there is usual weak spot for 3G. Line strength no problem with casing.
Will update here if i found another solution