ip.access voice traffic

Hello, Another beginner question. I have an ip.access that I'm using with openbsc. It's working, and I can make calls between phones, etc. When I capture a pcap I can view all the signaling between the ip.access and openbsc. But, I never see any voice packets. I suppose this could be because both phones are connected to the same bts. But, looking at some of the pcaps on the wiki, I do see that others have captured the voice packets. Is there a configuration option to force the voice data over the a-bis link? Thanks!