I've been researching this for a bit now, and I've yet to find anything to prevent a software solution to add this to N900 (Yes, it requires the AMR codec, but since N900 is 3G, it should already support that). I created a Brainstorm to discuss the implementation here, and a brainstorm discussion thread here. Please go to the Brainstorm page and give it a "thumbs up" vote if you'd like to see this happen!

I'm working on developing this on my own in my free time, but without active community or Nokia support, it will probably take quite awhile.