Ah, I got it working. I streamed via VLC using:
cvlc http://[shoutcastip]:[shoutcastport] --loop --norm-max-level=5 --sout='#transcode{acodec=ulaw,samplerate=8000,channels=1,ab=16}:rtp{dst=[PBXNSIP IP],port-audio=[PBXNSIP MOH PORT]]'
This is streaming an MP3 shoutcast radio stream to the RTP on Pbxnsip.
It seems to be streaming when I put myself on hold. It is however very choppy, chopping about every second or 2. I will try and run VLC on the same box and see if that improves the choppiness.
Any suggestions for streaming RTP with less choppiness are welcome.