I gave up on hacking for smartphones a long time ago. It's clear that they don't like me, so in turn I don't like them :)
This is because most phones are made cheaper using forced ads "pre-installed applications", rely on firmware hacks to get the hardware working, hold back updates so that you will buy a new phone, have to follow regulations on radio signals and should not be used for making free calls by hacking the SIM card. In my old job, I managed to hack the mDSP and see the wireless traffic being processed, but it took weeks to get the exploit working on a model built for firmware developers.
Making an operating system pretty much assumes that you're also designing own hardware. You need an SOC, a crystal oscillator, ceramic capacitors, a buck converter, a charging circuit, lithium-ion battery, a thouch screen, a CMOS sensor with Bayer filter, lens, shutter and ISP, a PWM or DAC controlled sound card, a speaker, a zener diode against voltage spikes, mini vibrators...