Has anyone built a solar powered cell phone charger?

Yes

I don’t know about their capabilities, though.

Great question.

Can you explain what you mean by “I want to know what the capabilities are”?
A solar powered cell phone charger can charge cell phones. With solar power. Most of them that I have seen use a mini-USB connector, though some have a set of swappable connectors. Available amperage (charge time) is dependent on the size of the collector and the intensity of the sunlight.
If you can provide more detail as to what capabilities you are curious about, we can probably give you more and better information.

Well I’m wondering how long would it take to charge a battery just like one in the phone to full charge?

According to CNET, this particular model takes 8 -10 hours of sunlight for a full charge. It probably varies by manufacturer. Size is an issue; most chargers would be designed for backpackers, and would be small enough to fit in a backpack. This would me that the surface area would be small, and the time for the charge would be longer.

I would be interested in one of those when I go downrange in the mountains. The towers can be hard to come by out there and so I think is the reason the phone battery seems to not last as long. There’s probably a catch, like the charger will cost way more than a spare battery or two.

@woodcutter, you probably need a satellite cell phone.

The amount of time required to charge a given battery depends on its capacity in Amp-hours (Ah) and its nominal voltage. For example, the battery in my laptop might be 3.4v with 1.4 Ah capacity. If it has 50% charge then it will need 700 mAh to bring it to a full charge. If the nominal voltage of the solar collector is 6.8v then I will need a transformer to lower the voltage to 3.4v, and which will give me almost double the available amperage (minus inductance and resistance losses). As I mentioned earlier, the available amperage depends on the intensity of the sunlight and the size of the collector, which is pretty widely variable.
@woodcutter You’d be best advised to take both a spare battery and a solar charger, given how long it might take to recharge (in case of overcast, solar eclipse, or what have you.)

Here is a terrific version of a vest with pockets to hold your stuff and solar panels to charge them while you walk around. Here is another site with a better picture of the back yoke (yolk?) where the solar panels are.

“Here” is a totally different winter jacket that operates on the same principle as the vest I mentioned above (see item number 7).

Yes. I’ve seen them on sale in the UK for about £10. On the other hand seeing as we only get six days of sunshine a year, I doubt they would be terribly useful over here.