Liam:
>Can you elaborate? Where I'm coming from is that some ISPs are offering
>"virtual PBX (centrex)" services and I'm trying to understand how far you
>could go with vPBX and a router before you need a hardware PBX.

As far as I know std. VoIP routers and ATA's have limits to how many ports
can be forwarded and generally let you set up one or two registrars. One of
the Sipura ATA's can be expanded to handle four VoIP lines. Beyond that you
will need to look at Cisco and other 'enterprise wide VoIP' providers.
Alternatively you could set up an Asterisk software on a Linux server and
equip it with FXS / FXO ports, but I am no expert on this.
You may get more info on comp.dcom.voice-over-ip usenet group where some
providers post their stuff.
Regards,
Martin