056 is the code for VoIP telephone number services. I believe these are generally charged at geographic rate, although they are more expensive from mobiles.

O2 charges them at 20 pence per minute for pay monthly subscribers and 15 pence per minute for those on pay as you go.

Vodafone pay monthly users pay "up to" 30 pence per minute whilst those on pay as you go pay 25 pence per minute.

Orange pay monthly customers pay (see PDF page 10) 20 pence per minute, but the pay as you go price guide overlooks the cost of 056 numbers.

Three charges for  (PDF page 19) 15 pence per minute for pay monthly and 10 or 15 pence per minute for pay as you go.

Alternative wrote on Dec 4^th, 2009 at 7:02pm:

I think that any move by the private sector to abandon 084 and 087 numbers in favour of 03 ones will take time. One reason for private companies not adopting 03 numbers is probably because there is little understanding of them by the public coupled with them charging what they can get away with.

The removal of the framework that supported revenue sharing on 0870 numbers has now been removed, so organisations no longer benefit in subsidies from callers as they did before. However, Virgin Media, the mobile providers and some other telephone companies still impose premium charges even though they do not pass them on.

SilentCallsVictim wrote on Dec 4^th, 2009 at 11:20pm:

Of those I looked at above, I am only certain that BT does include them in unlimited packages as they are in charge band g21, same as 03 numbers. On the BT Unlimited Anytime Plan it says UK calls includes g21 numbers.

Virgin Media says only 01, 02 and 03 numbers are inclusive. Of the mobile companies, O2 and Three specifically say that 056 numbers do not come from inclusive minutes.

Vodafone lists 056 on pages entitled "Premium rate call charges", so I read into that that they are always chargeable.

Similarly Orange pay monthly puts them in the "premium rate and other special numbers" group, so I assume likewise there.