Very few things are impossible given enough resources, but how much time and money are you willing to spend on these ?

1) In theory you could build a headset that connects via TCP/IP to a "server" running on N900 or Desktop machine or whatever which hooks into the relevant media frameworks to provide sound input/output paths. Requires plenty of electronics and embedded programming skills and a lot of time (if you have neither hiring someone to do the RD is of course an option if you're "independently wealthy")

2) I don't really know much about Asterisk, but AFAIUnderstand it should be capable of routing calls like this too. I have seen a package for it in some repo (extras-testing, maybe devel...) but have not given it a try.