Happy with Three. 3 to 3 call quality is FM radio standard. Feel at Home roaming is excellent for my needs.
2gig data per mth is fine for me at present and allows hotspot tethering for my tablet & laptop for work. Although I may evaluate a 2nd sim for the tablet one day, as it's slowly replacing my laptop. Less to lug.
My iPhone 5s also shuts down early when cold. As McKeano says, keep it warm in winter
I also have a 2-port Lumsing battery pack which acts as my portable Aggreko!
I'm also disappointed with the 5s overall battery life but I think that's as much due to OS & app "upgrades". I'll prob move to an iPhone SE at renewal in Nov.
However, I'm so impressed with my Sony Z2 tablet that I may try their equivalent phone for fun. I'm a bit of a Sony fan generally...