For the majority of users the minimum payment to BT is the line rental of �11.25 per month. You automatically get unlimited weekend calls but, if you're prepared to commit to a 12 month contract period, you can get unlimited evening calls for no extra payment:
http://www.productsandservices.bt.com/consumer Products/displayCategory.do?categoryId=CON-HOM E-PHN-R1
Those who are in receipt of Income Support, Income-Based Job Seeker's Allowance, Income-Related Employment Support Allowance or Guaranteed Pensions Credit can apply to use BT Basic. The cost is just �13.20 per quarter, including up to �4.50 in call charges. Additional calls to UK landline numbers cost roughly 10p per minute (plus a 3p flat charge per call):
http://www.btplc.com/inclusion/phoneservices/s ervices/btbasic/index.htm
Chris