Sorry, done to death I know but I imagine the answer changes all the time as new one come out.
I'm advising my 66yr old mother on her first smartphone. Simple needs- Phone, text, check email, take/send/receive photos of grand kid, maybe Facebook and maybe the odd train and flight times app. Really can't see it being more stretching than that.
It'll have to be Vodafone as that's the only network she gets reliable signal for, will need to be not too faffy as I'm 700 miles away so can't sort it for her too easily and after cashback/ redemption deals come in close to her current £15pm sim only deal inc phone.
What handset would you be recommending? iPhone fanbois here and the world of android looks full of options(and amazing vfm!)?