If I have the use of an office in, for example, the UK..... which is actually an IP phone,
Take the phone. Setup firewall to firewall VPN between your location and the UK. Job done.
I'm sure that there are other ways of doing it as well!
If the office has an SBC (session border controller) infront of the phone system, then theoretically you could eliminate the need for a VPN (and end up with a Vonage style setup). But personally, I'd always opt for a VPN ... it doesn't add much in the way of complexity, but adds a lot in terms of security.