0844-428-9090 is the access number that lets you call mobile numbers for 4p per minute. I thinks its provided by the same company that does 18866.

Clever Rates have now stopped the availability of letting you call mobiles via 0844-452-2525 and changed it to it an 0871 numbers which is charged at 6p per minute from a landline.

Note to Orange customers:

All pay as you go customers can pay 15p per minute to call non-orange phones. All they have to do is call 0844-428-9090 and enter the mobile number they wish to call. This call currently costs 15p per minute. The standard Orange rate for calling non-Orange mobiles without using the 0844 access number varies on the different tariffs which are shown below:

Same Rates 24/7: 35p per minute
Top up Text: 40p per minute
Talk and Save: 40p per minute
Choose your own off-peak: 25p per minute (Off-peak)
Choose your own off-peak: 50p per minute (Peak times)

Also note that from the 18th April 2006, the rate for calling 0500,0800,0808,0844,0845 and 0870 numbers will increase to 25p per minute.

Calls to 0871 numbers from Orange phones currently cost 35p per minute and will remain the same, calls to 070 prefixed numbers are 50p per minute.

The new minimum call charge for all chargeable calls will be 10p effective from 18th April 2006.