The "SoC" (System on a chip) in a Raspberry Pi is very similar to the chips that power low-end cell phones. It takes many weeks or months of development to get a new operating system to boot on one of those chips. Look at how long it took FreeBSD to boot on the RPi, and it still doesn't support 100% of the RPi hardware. And that was on a SoC that the manufacturer intends to be used in embedded systems, so they release full documentation for it.
If a real expert put the required time into it, and was able to get full documentation for the cell phone, they would probably be able to get FreeBSD to boot on it. The first requirement is theoretically even feasible. The second requirement is outright impossible for Apple cellphones (Apple will obviously not release documentation at the level of the SoC, since they manufacture it themselves), and I don't know what the situation for the better Android phones is (whether they use documented chips or not).
What are you trying to accomplish? If you want to run your own code on a cellphone, that's quite easy: Just write it for Android. The Android SDK is easy to download. Since Android is Unix-based, the basic code will be very similar to FreeBSD. For example, last fall I used a tiny little demo application written in Kivy (a graphical toolkit built around Python) on a Mac and on an Android device. Works.
Or if you are trying to use the cell phone network from a self-built device? You can buy cell phone "radios" with antennas that connect to systems like the Raspberry Pi. For example, Adafruit sells the "FONA", which is fundamentally a GSM radio modem that connects via serial port to a computer like a RPi.