I have here a D5503, no simlock (checked with *#*#7378423#*#*) on android 5.1.1. Completed a Checkmend yesterday and the device is clean which means no blacklist. The thing is even when it was on android 4 it had major signal issues, let me explain:- 1. Entered an EE sim card, it shows I have a voicemail but no service or connection. 2. Entered a vodafone SIM, shown one bar of signal I managed to use *#100# and retrieve my number, then I get full signal yet no access to network. 2. Later on with vodafone still full signal and no access to network I flashed the phone with an international ROM and United Kingdom ROM, same issue. Very frustrating, I also removed the back, screws and checked both antennas and they seemed okay. The ONLY message I have recieved is "Phone not allowed MM #6". But this points to being blacklisted, yet two IMEI checks prove it isn't, also more proof to me is that I could request number on vodafone which requires network access and EE showed that I had voicemails.