To protect the privacy of a user subscribing to a mobile telephony operator, the operator produces an isolating identifier by which the user can link up anonymously with a content provider. The operator/producer is the only entity capable of relating the isolating identifier to the user on request. An isolating identifier is, furthermore, either an isolating session identifier, hence one that changes at each connection of a user to a provider, or an isolating context identifier, hence one that persists over several connections. Furthermore, a context identifier is proper to a user/provider pair. This even further isolates the user. The format of the isolating identifier is compatible with the format of the NDS field, defined in the telephony standards.