You can send messages from your own number via iMessage, provided you don’t use the Auto Send option. If you do use the Auto Send option while you have the Premium plan, you’ll get a randomly assigned virtual number.

How do people know it's me when I use Auto Send?
You can add a signature (go to "Settings", then click on My signature) to make sure your contacts know who sent the message. However, users who purchase the Scheduled for Business plan get a virtual, fixed phone number with Auto Send. All your messages will then be sent from this number, always.
The recipient will reply to this virtual number. These replies show up in your app when you click on Received.

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