pager

US /ˈpeɪ.dʒɚ/
UK /ˈpeɪ.dʒɚ/
"pager" picture
1.

a small electronic device that receives signals or messages and then makes a noise or vibrates to tell you that someone wants you to contact them

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He carried a pager so he could be reached at all times.
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Before cell phones, pagers were common for urgent messages.
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beeperbleeper