My father in law uses one of my old batteries for a Nokia that has a solar panel on the back of it. He only turns it one when he wants to use it (don't you hate that, he calls us, we call right back and he already shut it off). Anyway it will charge it from dead in about 2 full sun days, or about 20-30 min a day to keep it topped off with his use. It isn't much larger then the regular battery.
If you’re not going to use the phone except in emergencies you could get a 9v adapter for it. Top off the phone before you leave and take a 9v and the adapter and you should be good for 2-3 weeks and a couple of calls.