If you have set the mobile # and the carrier, and also checked the "This customer consents to SMS messages", this action menu item will be available. e-manage|ONE uses Microsoft Outlook to send text messages to contacts, therefore you must have the Outlook Integration turned on in order to send SMS text messages. All three of these things must be true in order to send SMS messages.
Click the action menu item and the Send SMS Text Message window will pop up.
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Add a Subject and a Message and click the Send Message button.
Since e-manage|ONE uses Microsoft Outlook to send text messages, if the person you sent the text message to responds to the text message, it will come directly into your e-mail inbox.