A cordless phone with a minijack would definitely be an affordable option, and it allows you to do more than just answer the phone and you can answer without a lifter. Otherwise, everything we have here is Plantronics gear.
We have some that are basically pager sized analog phones (complete with dial pad) and a minijack for headphones; since they're standalone phones, no lifter is needed. The standard 2.4Ghz phones work just as well, but are huge in comparison. Our more common headsets are Avaya specific, which is the only option for a headset that interfaces with the phone and allows answering the phone without a handset lifter; these cannot do anything but off-hook/on-hook (i.e. no hold, transfer, etc...)
Besides possibly Bluetooth, I don't believe there is a standard for a wireless headset interface, so anything similar will be brand or model specific AFAIK. And Bluetooth will likely only transmit the convo, not interface with phone functions like answering; i.e. You would probably still need a handset lifter.
