Summary
To get someone's attention in a channel conversation or a chat, @mention them. Just type @ before their name and then select them from the menu that appears.
Note: You can also mention someone simply by typing their name. Start by capitalizing the first letter and as you continue to type the name, a list of people will show for you to choose from.
They receive a notification, which they can click to go directly into the point in the conversation where they were mentioned.
To get the attention of an entire team, try one of these:
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Post something in the Team's general channel. The channel name appears bold for every team member.
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Type @team to message everyone on that team.
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Type @channel to notify everyone who has favorited that channel.
FYI: This is enabled by default unless a team owner turns this feature off

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Reference Page Entry
Microsoft. (2020). Use @mentions to get someone's attention in Teams. Retrieved May 19, 2020, from https://support.office.com/en-us/article/use-mentions-to-get-someone-s-attention-in-teams-eb4f059d-320e-454e-b111-03361d4d6855#ID0EAABAAA=Desktop