Cheaper? Well you can get these well under $100 in "Refurbished A" condition... PC Liquidations, StartechTel, iStream, MetrolineDirect, etc... all carry these in refurbished condition from $55-$85 USD.
One reference, NOT necessarily a recommendation although we do buy a lot of used gear from them and have had excellent results, they carry the 6920 refurbished for $60:
https://www.metrolinedirect.com/mitel-6920-ip-phone.html The Mitel 53xx and 69xx series phones are MiNET devices (not SIP), a Mitel proprietary protocol, and other devices would have to be setup as generic SIP devices... Honestly, going to something like a low end Yealink will cost you the same or more than a refurbished 6920, and you lose a lot... mostly in what you can do with key programming and integration with the system, meaning no BLF appearances, no "one touch features", having to use feature access codes for everything, no receiving pages, etc.
