Ofcom's requirement is for call costs for numbers starting 084, 087, 09 and 118 to be declared such that you know the cost before you call. Call costs must be stated next to the number wherever it is advertised or promoted.
http://ask.ofcom.org.uk/help/telephone/businesscallingThere is no Ofcom requirement for call costs to be declared at the start of the call.
There is a requirement by the Phone-paid Services Authority (PSA) for some classes of Controlled Premium Rate Services (CPRS) to declare call costs at the start of the call. These are detailed in a series of special conditions.
http://psauthority.org.uk/for-business/special-conditionsControlled Premium Rate Services are defined in section 2 (e) of Ofcom's 'PRS Condition'. It is services defined as CPRS that are regulated by PSA.
CPRS regulation covers:
- unbundled tariff numbers with a Service Charge of more than 7p per minute or per call (087, 09, 118),
- all chatlines, adult entertainment and internet dialler-operated services, irrespective of prefix or price,
- numbers where the benefit to the called party and/or their telecoms provider is more than 10p per minute or per call (070, mobile sbortcodes).