to be held incommunicado
See: Incommunicado Not being allowed or able to communicate with other people. Any person arrested and held in custody has the right not to be held incommunicado, although there are exceptions to this. They may, on request, have one person known to them or likely to take an interest in their welfare informed at public expense of their whereabouts as soon as practicable.
(https://hmicfrs.justiceinspectorates.gov.uk/glossary/incommunicado/)