The 'BT Basic' deal has recently changed and is now a much better proposition.

Line rental is £5.10 per month including a £1.50 allowance to use against calls.

Calls to 01 and 02 landline numbers are charged 3.3p per call plus 11.3p per minute. This includes landline numbers allocated in CI and IoM.
Calls to 03 non-geographic numbers are charged 3.3p per call plus 11.3p per minute.
Calls to 071-075 & 077-079 mobile numbers are 3.3p per call plus 11.576p per minute daytime, 6.47p per minute evening, 3.406p per minute weekend.
Once you have spent £10 on those calls in a month (takes around 90 minutes or so), additional calls to those numbers attract no further charges.

Calls to 080 numbers are free.

Calls to 084, 087, 09 and 118 numbers have a zero Access Charge, you just pay the Service Charge. At weekends, calls to 0845 and 0870 numbers are free. You don't even pay the Service Charge.

BT Broadband can be added for an additional £5 per month.

The new version of BT Basic effectively gives unlimited calls to landline numbers starting 01 and 02, non-geographic numbers starting 03 and mobile numbers starting 071-075 and 077-079 for £15.10 per month, with a smaller bill if less than about 90 minutes of calls are made in that month (and a £5.10 bill if less than £1.50 of calls are made).

Paying £15.10 per month for BT Basic is nearly seven pounds per month cheaper than BT Home Phone Saver (HPS has unlimited anytime 01, 02 & 03 calls and 7.5p/min for calls to 071-075 & 077-079 numbers). Additionally, the Access Charge for calls to 084, 087, 09 and 118 numbers is zero on BT Basic as opposed to 11p per minute on BT Home Phone Saver.

BT Basic is now a serious tariff, offering a useful option for those with a limited budget. A "line rental plus free weekend calls" deal (with expensive weekday calls) is now seen to be the wrong tariff for almost everyone.

BT Basic is available only to people who are in receipt of certain benefits.