This "ICAO licence" is almost completely meaningless in any event. All but a handfull of very small, mainly caribbean nations
are ICAO member states. In any event ICAO does not issue pilots licences. There is no compelling evidence to suggest that those few small islands that are non ICAO signatories issue pilots licences either.
As has already been pointed out, there is no automatic interchange between licences issued in one state for that of another. Any acceptance or conversion requirements depend on the requirements of the state to whom the application is eventually made. Obviously countries and supranational authorities with a large aviation infrastructure including flight training will provide the majority of licences issued. At this time that is probably the FAA in the USA and its territories as well as the European JAA member states. However countries such as Australia, New Zealand, Canada, South Africa will also contribute a significant number. To perhaps a lesser but increasing degree, other signatory nation states are also developing training institutions based on either their own, or foreign licensing requirements and standards.
An extract from the ICAO website:
ICAO does not issue any licences. Licences issued by ICAO Contracting States on the basis of Standards and Recommended Practices of Annex 1 – Personnel Licensing, are habitually called ICAO licences. This has led many to believe that there is a specific ICAO or international licence. The fact is that there is not one single international licence issued by ICAO or any other organization. States issue their own licences based on national regulations in conformity with Annex 1 specifications and validate licences issued by other Contracting States on the basis of bilateral or multilateral agreements or the fulfilment of nationally legislated requirements.
Any pilot who wishes to fly on an aircraft registered in a State other than the one that has issued the licence, needs to obtain an authorization from the State of Registry. This authorization is generally given by the State of Registry through a validation or a conversion of the foreign licence. In general, the validation process is used for short-term authorization while the conversion process is used for longer-term authorization.
Validation of a foreign licence
When a State validates a foreign licence, it recognizes it as valid for use on aircraft on its own registry. The Convention on International Civil Aviation and its Annex 1 do not contain specific requirements for the validation of licences beyond establishing the principle and the fact that the validity of a validation, cannot be extended beyond the validity of the supporting licence. As a result, conditions under which validation is granted vary from State to State. It depends on the level of privileges required and on the origin of the licence. It is generally easy to obtain a validation for VFR private flights, but more stringent rules may exist for professional licences. The applicant may be required to get additional training and/or to take new exams.
Conversion of a foreign licence
As an alternative to validate a foreign licence, a State may issue a licence that is based on the foreign licence held by the applicant. In doing so, the Licensing Authority accepts the fact that holding a foreign licence is an acceptable way to demonstrate compliance with its own national licensing regulation. The conditions for the conversion are generally similar to that of a validation.