I noticed that H.264 is disabled in the App Store build, most likely because Linphone relies on the GPLv2 x264 library (and thus cannot be distributed on the App Store). In the upcoming iOS 8 release, Apple finally made public their VideoToolbox.framework, which provides more direct access to the hardware H.264 encoder/decoder. Now developers no longer have to rely on horrible hacks to get their realtime hardware accelerated H.264 video! This same framework has been available on Mac OS since 10.8+ as well.
I haven't investigated how easy it will be to integrate into Linphone yet, but here is some sample code demonstrating how to use VideoToolbox to encode frames in realtime:
https://github.com/jgh-/VideoCore/blob/master/transforms/Apple/H264Encode.cpp
Cheers!