I have to first ask is it not the whole point of doing such a thing? That said I purposely lack a phone and have no idea what it is like to be attached to one. Also are you making camp somewhere or carting it around with everything on your back? If it is all on your back you really do not want to be adding too much extra.
Are we talking emergency use, a bit of light texting in the evening or chatting 24/7 in "I think you missed the point" style?
Most modern phones charge by USB (I think it is EU law even) so you do not have to refine your search beyond does it have a USB. Knowing this most things will chuck a USB onto the device as a little extra selling point.
Your three options are mainly
Solar. I mostly deal with 12V or full on house stuff and have not really looked to see what is happening at the 5V world. Similarly at this time of year it is probably not worth the hassle, especially if you have to carry a battery too.
Wind up. I would probably hang it off the back of a wind up torch.
Battery. There are things like the minty boost which will be the easiest though the wind up stuff I am looking at.
You say reliable.... proper climbing or milspec grade stuff will come at a reasonable price. Your basic wind up torch or the other options seem to run you about £15. That said the climbing/military stuff will be designed to take a pounding, the other stuff is hardly unreliable though.