It's from a Finarea company called Phonecheap and costs 5 pence a minute from a BT landline. Other cheap calls operators cost from 6 pence a minute upwards, via either 0871 numbers or carrier-select prefix code such as 18185.

I have no idea which foreign companies you can call these numbers from, but doubt it is very many. And there really is little point anyway, as you will have other options to reach UK mobiles such as VoIP that you already mentioned - as RT says, voipcheap.com is amongst the cheapest, at 7c/min + vat

Right, I'm using voipcheap.com, but there is most of the times a delay, resulting in a prehistoric walkie-talkie kind of behaviour :-)
And I was hoping it would be another company also offering VOIP.
Anyway, it's cheap, and you get what you pay for.