It will depend on the type of pV cells used. Monocrystaline will deliver loads of charge in bright sunlight but as soon as there is a hint of cloud, they aint worth anything. Polycrystaline deliver a smaller charge but will always produce something if there is light, even under a cloudy British winter sky. Monocrystaline are cheaper so guess what manufacturers tend to use? Obviously, correct positioning with regard to the sun is necessary for efficiency.
If you have a Lithium Ion battery, (as most modern phones do) then it will hold less charge generally but take small top ups without ruining it. This, combined with the higher demands of more processing power in smartphones is why modern phones don't last long. Older phones tend to have Nickle Cadium batteries or similar, these will hold more charge for longer but abuse them at your peril, they need to be fully decharged and then fully recharged, any small top up or fully charging from half way will ruin the battery and will need constant charging.
If you are in a remote area and there is no signal- switch off your phone. It will constantly be looking for signal which aint far behind making a call regarding battery demand and you will run out very quickly so when you do find that magic signal, you have no chrage left and it dies on you.
Spare phone batteries don't cost nor weigh much, thay also hold their charge for a very long time if wrapped in plastic (a few layers of cling film does well) or another good insulator (on the same note, don't store car batteries on concrete or the metal frame of a generator, they will decharge reasonably quickly, put them on a block of wood or another good insulator and they will last alot longer- ever noticed how those emergency car start devices are encased in thick plastic? This is the reason.)
I am regularly on the road for weeks at a time, I use an old Nokia (don't know the model) and two batteries which have been charged properly every time so still have lots of capacity, the phone gets switched off at night (if I remember, often it doesn't), my record is three and a half weeks before needing a charge but that was in a bad signal area so there was alot of down time for the phone. I have dabbled with wind-up chragers and all they did was wind me up, you need to get the speed just right and it would take hours to get a full charge off one; also, loose the right connector and you're up the creek proverbial.
There are plenty of expert outdoors folk who now rate a mobile phone as more important than a whistle in the woods/mountains. A small spare one in you kit would not even need to be registered nor have any credit on it to dial 112 or 999 as a back-up to your regular one that will probably run out, especially in a crisis. Then again, it still won't help you if there's no signal and you didn't tell anyone where you were going, your route and when you intended to be back; the oldies are the goodies.
