You have to determine what state the call was in when you hear the dual ring tone.
When the call is in "180 Ringing" state, CUPC will play local ring tone. At this point, there should be no RTP stream at all.
After CUCM sent "200 OK" to CUPC, CUPC will send ACK. After that, RTP stream is set up.
If CUCM sent "200 OK" before PSTN call was connected, it's probably by designed.
You may open a TAC case to verify.
Thanks!
Michael
http://htluo.blogspot.com/