In short, 0845 and 0870 were smuggled through parliament into the "big number change" due to heavy lobbying by BT.
They are called "Non-geographical, revenue-sharing, non-discountable" numbers.
They PAY the recipient, so call-centres have an incentive to keep you in their queue - this has even cost call-centre jobs.
In June 2006 the Department for Work and Pensions was castigated by the
Parliamentary Accounts Committee for using an 0870 number to collect
over £3 million from job-seekers. They complained that an
0800 (freephone) number had cost them £10m, but it was pointed
out that was over an 11 year period, while the £3m had accrued in
just 15 months. They have changed the number now to an 0845 but it
still taxes the unemployed.
The "big number change" was supposed to clarify charging by
making "08" numbers "cheaper than normal" and "09" numbers "more
expensive than normal", but the 1999 Act fixed 0845 and 0870 at the
THEN local and trunk rates. These have since fallen considerably, so
both are now certainly "more expensive than normal".
These numbers are "non-discountable", so that they cannot be included
in "monthly flat rate" call schemes. That makes them even more
expensive.
Links
Say No to 0870
Offcom's toothless attitude