Since nearly all "smart" phones are ARM based, someone would need to port FreeBSD to that platform. OpenBSD and NetBSD have more support for ARM based devices. On those systems, one uses the platform closest to what is needed and builds from there.
There have been builds for pxa-270 based boards. HTC Apache and other older devices may be a good start.
The trouble would be with porting the telephone interface to FreeBSD. OPIE, GPIE, and Matchbox have either been ported to FreeBSD or can be built from source. With the OPIE and GPIE, I'm not sure if they have a dependency on Xfbdev.
Even though FreeBSD is more stable and "faster"- this is my opinion and observation- you have to deal with the fact that people don't usually want to start on something new.
