The alarm test on 23 April will be heard in the UK only.
Channel Islanders won't hear a nationwide emergency alarm from their phone on 23 April.
A majority of UK smartphones will emit a loud noise and vibrate at 3pm on Sunday 23, and any islanders in Britain will also get that alert.
The alert would only be used for real if there was a national emergency, or a localised threat, like extreme weather.
The islands' emergency planning officer, Kevin Murphy, says they have local solutions, should there be a need to warn islanders of an emergency:
"Both Islands have various options for alerting Islanders of an emergency at short notice. In such a small community, in many cases alerts would be issued effectively through traditional media and social media.
We will continue to review our options for how we communicate with the public in an emergency."
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