Any call terminating telecoms company that reveals withheld CLI to its clients is breaking the law. Whatever the arguments about the value of this information to taxi companies, the caller's wish to withhold CLI MUST be respected.
Regulation 10 of the PECR covers the right to withhold CLI - free of charge.
Regulation 13 of the PECR requires the call terminating operator to co-operate with the call originating operator, by ensuring that a request to withhold CLI is respected.
Regulation 14 of the PECR covers the processing of "location" data, as referred to in Reply #4.
There may be technical legal issues over the manner in which "consent" to use this data is granted in some cases, however I would suspect that anyone calling a number advertised as only being usable from overseas would have a problem in getting action taken over a case where their misuse of such a number (to call from the UK) was detected without their consent.
The ICO is strictly responsible for enforcement of compliance with the terms of the PECR, as this is technically classed as a piece of Data Protection Legislation. Ofcom is however primarily responsible for the behaviour of telcos through the imposition and enforcement of General Conditions.
General Condition 16.2(b) (on Page 56/95) requires: Calling Line Identification Facilities to be provided “
in accordance with the requirements of Relevant Data Protection Legislation”. A breach of the terms of such legislation would represent a breach of that General Condition.
If anyone has clear evidence of a breach of any of these regulations, the fair telecoms campaign would be delighted to support them in getting action taken.