International calls
If you make any calls internationally (or even if you don't) you can set up one or more outgoing lines via the Internet. So instead of a call going from an extension into the PBX and then into the phone system, it goes from the PBX over the Internet to a "VoIP termination" where the call transfers to the public phone system. The people who run these services buy a lot of phone minutes, so they get a very good rate. Instead of paying maybe £1.50 for a ten-minute call to the USA on a standard BT line you can get the same call for around 15p.
Virtual international offices
If you are doing business abroad then you'll know how important it is to have local contact details. But the cost of getting a local office manned and set up is often very high. Now there's a really cost-effective alternative to having a real office in the region - just get a phone number in that region that connects back to your head office.
This is all done by VoIP. You can purchase a number in, say, the Silicon Valley area of the United States with the 408 prefix and any calls made locally to that number will be passed over the Internet and will end up inside your PBX. You can then decide what to do - such as dialing the technical support extension but with the Caller ID name set to "From the US". You can have numbers all over world at very reasonable prices.
