It sounds like a missed call alert. If you were out of coverage, or your phone was switched off when someone tried to call, that'll be why your phone's call log doesn't have a record of the missed calls, but we send these by text to let you know someone tried to get in touch.
You can switch missed call alerts on and off, from your Voicemail service by dialling 123, then it's option 5 on the main menu.
Thanks, Jonathan
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