It seems that I have managed to fix the problem-the solution might be useful for other people with this problem(It seems that it's a common problem with W910i). I've opened the case to check if the antenna cord is ok-I've unplugged it and then removed whole internal antenna. I didn't notice anything unusual, so I have put it back together and close the case. After that I'm not loosing my signal constantly and not getting any "Emergency calls only", it's been 3 days now so I guess that it was something with coax cable after all. Apart from this, phone got lower signal than other mobiles, that I own, but apparently that isn't anything unusual for W910i.