I bought the PA17-79 a few years back and it's a piece of junk.
It's a very long story, running to dozens of emails and faxes, and eventually I was able to return it to where I bought it from (a U.S. mail order outlet) for a refund. In the process I discovered that every advertiser was putting out bogus attenuation claims on this headset - 44db I recall - following misleading figures from the manufacturer. I have graphs of my actual headset showing a far lower figure and a confirmation from the mfg that this headset is in spec.
Mine whistled a little but the worst bit was that the attenuation was poor.
Then I did a test of a load of headsets, bought a Bose X and never looked back. Just make sure you get one of the current model ones; Bose were selling duff mikes for a few years and the amnesty for getting them exchanged has now expired. They still worked OK, better than most headsets in fact, in most (but not all) intercoms.