Savings depend on when and how you call and how you pay. The cheapest phone services require up-front payment by credit or debit card. Larger up-front payments can attract discounts or free minutes. A smaller number of companies work by sending out a bill, just as BT does. In each case it pays to check the small print before signing up for services.
Phone companies have different rules for peak and off-peak rates and for how they time calls.
BT has rates for daytime, weekend and evening calls. Most of the pre-pay phone companies have a single rate that applies around the clock. BT charges for calls by the second but has a minimum call charge of 5p. Other companies' charges vary widely. The best savings come from switching phone companies for international calls.