Calls are normally charged per minute and there are no peak of off-peak times. Global One has just brought out a flat 29p rate for calls made to the UK from six European countries and America. The best-known card, the BT Chargecard, is vastly more expensive than the others. Watch out for high rates using calling cards for making calls within the country you are visiting. For example, with the BT Chargecard a minute's call within America costs 1.50 and within France 1.14.
Some companies also sell prepaid cards through tour operators and travel agents.
Drawbacks are that you can end up with unused units and they can be less good value than account-based cards. Companies are not keen on being specific about how much it costs. First, you need a phone that will pick up the different radio frequencies of the networks in other countries. Then contact your mobile phone company a week or two before you go. You may want to set up a voicemail arrangement - otherwise you'll be charged international rates for calls received by your mobile while abroad.