I am not certain that the available surface area of the phone would be sufficent to charge the battery. Further, most people are using their phones during daylight hours where you will find the device tucked away in a dark pocket, handbag or carrying case- no sun available. There are options. I have seen those new fangled flash lights that you shake to charge. They will stay lit for a short time and then need another shake. Perhaps you could have a phone that you could shake when you desired to use it. Given its short charge life it would be ideal for calling one's mother-in-law.

For backpacking this idea might work nicely. For emergency use the backpacker could strap on the battery pack onto the backpack frame and have it charge while they are walking around. But it would be too costly for manufacturers to make these special units available for the small number of backpackers concerned about taking a cell phone.
Better would be a solar power panel with a 12v output and use the 12v charger that comes with the cell phone.

You can get solar bacpacke, or portable panels that would actually do good to charge phones.