In order to use a callback service, a subscriber is allocated a unique number in, for example, the US, which must first be dialled in order to trigger a return call. This is known (in the US) as a Direct Inbound Dialing (DID) number, or in the UK as a Direct Dial-In (DDI) number. Where Caller ID is available, it may be possible to use the same DID number for many different subscribers, as the callback system will be able to recognise each subscriber's registered number.