As part of a lab project (Roaming PBX), I have compiled SILK and AMR codecs for Asterisk. The AMR codec compilation was based on online research, while SILK was compiled following the instructions in this repository.
Update (March 20, 2013)
I tested SILK8 (SILK-NB) on Android devices using CSIPSimple and Bria. The performance was not optimal, and the quality was definitely not fantastic. This was true for both my self-compiled version and the official Digium SILK codec.
Currently testing AMR-NB.
Update #2 (March 21, 2013)
SILK8 performance with Bria on Android is acceptable after modifying codecs.conf with the following settings:
[silk8]
type=silk
samprate=8000
fec=true
packetloss_percentage=10
maxbitrate=20000 ; Default value was 10000
dtx=false ; Encode using discontinuous transmission mode or not. Turning this
; on will save bandwidth during periods of silence at the cost of
; increased computational complexity. Off by default.
I encountered some minor issues with AMR-NB and will post an update later.
Update #3 (April 15, 2013)
AMR-NB performs significantly better. I plan to describe the installation steps in a separate post. The quality is amazing, and the bandwidth consumption is very low. Unfortunately, due to licensing issues, AMR-NB cannot be included as part of the official Asterisk distribution.