Set up RTP listening ports without setting up calls

I’m trying to open multiple listening RTP ports on pre-determined port numbers from within Asterisk without setting up SIP or H.323 calls, to receive incoming RTP streams. We demonstrated it can be done by placing a dummy sip call into Asterisk, so the ports would be opened, but want to not rely on an external application. The most obvious method appears to be to originate a channel.