It's not as cheap as VoIP, but instead of having to add credit that may expire after a period of inactivity, you're only charged at the end of the month for each call you make (the rate for that country, plus a 4p (5p for UK numbers) connection fee per call). (moneysavingexpert.com)
Some airlines do expire miles after a period of inactivity. (creditdonkey.com)
You can save power by having Windows dim and then turn off the display after a period of inactivity. (thewindowsclub.com)