With quality speakers and microphones (or a headset), you can make calls from your PC to a landline and your recipient would not know that you're using your computer to call. There is free software that you can download that let you make PC-to-landline phone calls or special units that you can purchase that will allow you to connect a landline phone to your computer.
Contact your landline phone provider or wireless provider to see if they have any additional add-on options that allow you to make PC-to-PC calls. For example, AT&T U-verse's 2wire wireless router has a port for a telephone to make landline phone calls.
Download a free PC telephone service. Google Voice provides you with a unique number that can be used to make and receive calls either from your cell or landline phone or through your Gmail account. Access google.com/voice and gmail.com to set up a new account. Once it's established, access your Gmail account and click "Call Phone." Enter the number of the landline phone you want to contact and click the green call button.
Purchase a telephone adaptor service like MagicJack (approximately £25 as of April 2011). You'll be able to make unlimited calls to landline phones from your PC. Connect the MagicJack to an available USB port of your computer and then connect your landline phone line. Make a call on the phone like your normally would.
The faster the Internet speed package you are paying for, the better call quality you will experience.