Multiple VPN-Interfaces and intelligent routing

I need to explain our network-configuration

About 100 computers build up an 192.168.44.xxx network, there is a
vpn-gateway, to get online every user has its own vpn-account (set up as
dial-up-internet connection) - simple.

Now some of us use ip-telephones, those are not able to get online via VPN,
they need internet "directly"
All of us who are running linux, can simply set their pc's as gateways for
the telepone and forward from VPN to the telephone.

As you can imagine, not ALL of us use Linux, therefore we want to set up a
server.

As told before, the linux-computer logs on via my vpn-account, there is no
special ip-telephone-vpn-account. If all the windows-users set there
telephones onto that new gateway (my linux-computer), they all use MY
vpn-account (NOT GOOD, we got traffic-limit)

My Idea is that my computer creates all the vpn-connection needed (i.e.
VPN01-VPN15) and links each telephone to "his" vpn-tunnel.

Lets say the telephone from user5 is connecting to the gateway, it creates
the vpn5 connection and routes everything from that telephone (but ONLY from
that telephone, via ip-check or so) to VPN5 (static IP btw), and back again.

At the same time, user10, having my computer set up as gateway as well,
tries to make a call, so his request should be routet out through the
vpn10-tunnel

There is only ONE real network interface build into my computer (the one
connecting to the lan)

Is that possible? Thank you for your help!
John Bieling