They call that a callback service and they are very common.  In England, you
pay significantly more to call a mobile phone than you do to call a
landline.  A call to a mobile can often be 25p a minute.

The cheapest callback service I've found for the UK charges 9p a minute to
originate a call, but on most plans you can get rates as low as 5p a minute.
On international calls, a callback service can sometimes save you money, but
not on domestic calls.  A callback service also can save money for users of
some low minute plans and/or some prepaid sims.  It also saves money on
roaming charges because most UK providers charge a different rate to receive
calls abroad than they charge to originate them.

A friend of mine was thinking about creating a callback service specifically
targeted at Nextel users (who have free incoming on many plans).  He went so
far as to set up a beta test, but decided that the cheapest he could offer
calls for was six cents a minute (remember he is paying both legs of a call)
and that most people wouldn't screw with that to save a few cents.  There
was some potential for people who were over their allotted bucket of minutes
and for international calls, but he didn't think they would make more than
$3 or $4 a month per customers per month and decided that he could not
justify the operation.

Stu