Indeed. Pretty much all the official guidance now is to specify the actual call costs (in terms of a BT landline).
(Of course, when the official
BT pricelist name for the tariff most of 0870 is charged at is "National NTS" then no wonder it still gets used!)
You could of course tke the view that using a 25p/call 09011 is cheaper for the customer than a 10p/min 0871 if the call is going to last more than a couple of minutes... But THAT only goes to show how well 0871 deceives most people...
One phrase I've not heard for quite some time in relation to call costs is "less than the cost of a first class stamp". I guess that's because mostly, nowadays, they aren't ... It used to be common though, when the usual route of entry was to write in..