Hints for getting the best value from this service
BT's standard per minute peak charge for 020 7124 6666 calls is 4p from within the 020 and surrounding local call area exchanges, but 8p from the rest of the UK. BT phone call discounts will apply if you are on any of the BT Together packages.
BT charges the standard local rate (4p at peak times) for 0845 451 6666 access from anywhere in the UK, with no discount available on any package.
The 3p per minute surcharge for using 0800 594 6666 is deducted from your available call credit.

CPS services, mobile phone packages and cable rates vary: check with your provider for the cheapest access number.

What does all this mean? Or, which is the cheapest access number?
If you are on BT's Low User Scheme or standard/business rate package: use 0800 594 6666 if phoning Monday to Friday between 8am and 6pm and 020 7124 6666 (for London) or 0845 451 6666 (outside London) at any other time.
BT Together packages make 020 7124 6666 the cheapest access number.

If you have CPS through another phone company and still pay line rental to BT, you can use BT's cheaper rates by prefixing the top up and access numbers with 1280 to route the call to BT.