Cheapest tariffs and other value deals are out there, Monthly line rental under £15 to £20 contract deals are ideal for people with low monthly usage. Cheapest deal monthly tariff for less than £15 to £20 are available on 3, O2 offers a range of online Simplicity tariffs, including Monthly line rental under £15 to £20 deal. T-Mobile also offers an 18 month contract for £15 to £20 a month. The cheapest deal from Orange is called Dolphin 15. You can to sign a 24-month contract; you'll receive 100 free minutes and unlimited texts for £15 to £20 a month.