the reporter wants to remain anonymous but I need more information, is that allowed

Follow-up questions are allowed, even with anonymous reports

Yes, that's permitted: asking follow-up questions is separate from the right to remain anonymous. Anonymity means the reporter's identity is not known or disclosed, not that no further contact is possible. At signalo, that contact takes place via the case code: the reporter logs back into the reporting environment using it, sees the handler's question, and can respond to it — still anonymously. So no name, email address, or phone number is needed to continue the conversation about the content of the report.

What is not allowed is to use anonymity as a pretext to ask for identifying information anyway, or to give the impression that providing more information is only possible if the reporter identifies themselves. So ask for facts, dates, names of people involved, documents, or witnesses — that is standard investigative information — but not who the reporter is, if they have not chosen to disclose that. If an employee loses the case code during this process, there is a separate route to restore access to a report without breaking the anonymity. And because the clock for the statutory time limits keeps running while follow-up questions are being asked, it is important to know when the reporter must be informed about the assessment.

Where this comes from: directive and confidentiality obligation

Directive (EU) 2019/1937 leaves it to the member states whether to require anonymous reporting or not (article 6), but as soon as a reporting channel facilitates anonymous reports — as signalo does — that does not change the confidentiality obligation regarding the reporter's identity under article 16 of that directive. That confidentiality applies fully throughout the entire process, including the asking and answering of follow-up questions. The Whistleblower Protection Act elaborates these principles for Dutch practice.