From aa-asterisk.org.uk wiki
Non-geographic numbers are those numbers which do not implicitly have a location. In the UK National Telephone Numbering Plan, this is any UK number that does not begin 01 or 02. Note that 01/02 numbers supplied 'out of area' using (e.g.) VoIP are not classified as non-geographic.
Mobile telephones are also explicitly excluded from this definition. These are the numbers beginning 071 to 079.
Typically non-geographic numbers are either UK-Wide numbers, freephone numbers, or business rate services such as 084X/087X. Some non-geographic numbers can be classed as NTS, which means that the number can be routed by the exchange equipment to a landline with a geographic number instead, which can be changed as required. However non-geographic numbers are not required to be NTS even though many numbers that are not mobile numbers are.