The step up converters are DC-DC switching regulators which is the most efficient way to achieve what you want to. Most phones run at 3.3v or less nowadays, so you are unlikely to find anything useful in there. Even if you do, they will work on the same principal as the ones Sparkfun and others sell.
I am guessing from the voltage that you are using either a Li-Ion or Li-Po battery, in which case something like this will work well whilst also protecting the battery for being over discharged: