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Select 'Send SMS' from the list of available post functions. 
Fill in the message you want to send (see replacement variables to add issue-based information), and select the destination to whom you want to send the text message. You can choose between: a user defined in the issue (assignee, reporter, custom fields) each user in a group defined in the issue (custom fields) a fixed user each user from a fixed group * a fixed phone number 
Press add, and publish your workflow, and you're good to go!