Old USB ports would at least give .5A, and form what I can tell if you can chareg other phones it can do that. As the USB gives out 5V and the battery requires 4.4V it should take twice as long as the battery Ah to charge. I.e. with a 3000mAh (3Ah) battery in the fairphone it should that just over 6 hours to charge (given efficiency losses).
You can of course put the phone in aeroplane mode to ensure there are no radio drain, but it should charge.