CRTP works by compressing the IP and UDP headers. That means it cannot be used over mobile networks.
The audio is already compressed so cannot be compressed further - RTP is as light as you can get while still being routable via the Internet.
You should compare the different codec options. There is a large difference in the bandwidth use of different codecs. Eg compare G711 vs GSM, CELT, Speex etc.
Other advice: Increasing the ptime will fit a longer period of time into a single packet so reduce the IP/UDP/RTP header overhead. Also increasing the jitter buffer length may help compensate for the heavy jitter you may see on a mobile connection.
Steve
RTP's head is too heavy for mobile network. There is little about CRTP on google, anyone use it?