In the past, the list of 0800 numbers that qualified for free calls from mobiles was IIRC quite limited: Childline, Samaritans, several other charities, and perhaps the AA (or was it RAC?) and very few others.


Nowadays that list is quite long and, at least for Vodafone, includes job centres and others:

http://www.vodafone.co.uk/cs/groups/public/documents/webcontent/vftst004274.pdf