Xperia Z2 :: Phone Says No Signal Most Of The Time

I am an avid sony fan and I really love my xperia z2 except that I can't use the phone for its main purpose: to text or call or contact someone. It often says 'no service' if not 'emergency calls only'. On the first week, it's not that worst but now almost anywhere I can't contact, except when.in very high open places. My friends have signal bars whereas I have none. I tried changing sim and even factory data reset (as told by the shop) but I don't think my problem was solved.