if the phones are registered to the cucm properly, with correct MRGL's you should collect sdi/sdl traces and sniffer captures when the issue is happening
also what kind of conference bridge are you using, hardware/software?
1. after getting the sdi/sdl, you need to check and see that the correct CFB gets used, and check the traces at the time when the three beeps are heard to see whats happening,
2. check the sniffer traces from the phones that have the problem and the CFB
thanks,
Nart