Look for some system you plug the 2.4GBz 'base' into the voip box. That works for 900 or other MHz as well as standard phones.
NO ONE will make one that will talk directly to the cordless handsets.
I recently had to cancel my subscription to AT&T CallVantage when my D-Link router (supplied by AT&T) shut down and all I got when I picked up the phone, was a constant busy signal.
I've heard there are routers available that are compatable with the digital phones, with the VOIP service. Suggestions please??