RTP traffic and nf_conntrack_sip

Hi, searching ML archives I found that "RTP can be matched on using "-m helper --helper sip", so I tried the following simple rule on my router:

iptables -t mangle -A FORWARD -m helper --helper sip -j CLASSIFY --set-class 1:1

but it doesn't match any RTP traffic (no packet classified). I tried different nf_conntrack_sip module parameters but it didn't help.

Moreover, the nf_conntrack_sip (the module that should match SIP & RTP protocols) is not loaded by iptables.. so now I wonder how does nf_conntrack_sip work. Any idea? Am I missing something?
Thanks,