VoIP wired router behind a wireless router and AP

Hello,
I recently set up a wireless network connecting to an FTTH cable modem I had
installed at a friend's home 4 km away. The FTTH modem connects to a
802.11b/g wireless Buffalo router. The router connects to my home network
via 2 antennae, a Buffalo access point, and hub. I'm getting about 2mb
throughput to the internet from home. For my little knowledge of wireless
networking, it seems to be working alright. When I signed up for the FTTH
service, I also signed on for the IP phone service being offered. The FTTH
provider sent an rental NEC VoIP-enabled wired router (I live in Japan) and
the usual setup instructions explaining that the VoIP router is to be
plugged directly into the FTTH modem, which is 4 km away (my home phone and
phone line, of course, are supposed to plug into the VoIP router). Instead,
I'd like to try to set up the IP phone with the VoIP router placed at home,
behind and connected to the access point. Is this possible? Can someone
perhaps give me some general instructions for setting it up?

Any information would be appreciated.