Welcome to ETO!
Doesn't sound doable to me, using readily-available parts. How much space do you have available inside the phone? How will you prevent the phone from responding to the generated 5V by trying to charge the battery which is driving the boost converter?
A Typical Switching BOOST Supply uses a Choke, that has Too Much HEIGHT.
Possibly the Only Option would be a ICL7660 to Double the Voltage, Followed by a 78L05 Volt Regulator.
But The parts would Need to be Surface Mount, to keep the Height Low.
And you better have GOOD Soldering Skills.