On UAL, it is USUALLY on Channel 9, but on some aircraft (747?) it is on another channel. They are the ONLY carrier that does it and they do it through the third radio, which is normally used for ACARS, too, and not used by the crew for voice comm.
Some Captains don't like it, because some passengers seem to think it is their window into the front office and entitles them to criticize phraseology, a missed radio call, a frequency error, etc, etc... In addition, because it taps into the voice comm on VHF1, if there are any irregular calls between aircraft and ATC, it opens things up to misinterpretation, etc.. Captains often brief turning it off as part of the pre-flight briefing of engine failure on takeoff, but it is not always the first thing on most people's minds when it hits the proverbial fan.