I've been using the freely downloadable Skype for a little while now to make free computer to computer voice calls, using a [Trust] headset.
Today though I paid the £10 + £1.50 VAT for Skype credit giving me their current special offer of being able to Skype UK landlines for free for 6 months. The credit I can use for calling mobiles, texting mobiles, calling international, and call forwarding charges (I've set it up so that if I'm not at my computer, calls are forwarded to my mobile, and I pay for the call at Skype's rates which are cheap).
I also paid £20 + £3 VAT to get a Skype phone number for 12 months, allowing people to phone me on Skype. This also provides me with a free voicemail service.
For credit top-ups I have Paypal set-up to allow me to top-up in Skype with just a couple clicks.