Searching ip_conntrack_sip only brought up rather old and unhelpful results. It has been renamed nf_conntrack_sip.
Searching for nf_conntrack_sip brought up this wiki article from FreeSWITCH containing a nice explanation: https://freeswitch.org/confluence/display/FREESWITCH/Firewall
So: it is a non-mangling ALG (good - different from the NAT module) and requires standard ports and also for the kernel module to be loaded. You can use non-standard SIP ports with the module and tell it whether to work with media only coming from the signaling proxy or from anywhere.
It does not simply open 10000-20000 UDP for rtp traffic but examines the SDP to open the correct port.
On a FreePBX Distro this module is not loaded by default. On my own router this module is not loaded by default. The module looks helpful but is not necessarily automatically available to help with RTP ports–you would have to be sure to load it and possibly add extra options for your environment.
Thanks for the new information.