We use a Service provided by InternetCalls.com
With their software you can call any UK and Many Overseas Landlines for Free.
You need to visit their web site and download their software.
You may need to purchase a better soundcard if you are having trouble getting people to hear you through your computers microphone.
This is because many PCs have budget soundcards or use integrated soundchips rather than a better quality soundcard like a Creative Labs Soundblaster Live 5.1
You can then either use the Computers Speakers and Microphone to make telephone calls or you could plug in a Headset and Boom Mic if you want to make private calls.
Using this system save my Company about £50 a week in telephony costs and it doesn't tie up our phone lines for incoming calls.
If you have any further questions then feel free to visit our eBay shop : Highridge Computers Ltd.