Send messages on workflow event

As a JIRA Administrator, select "Issues" in the admin dropdown menu and then navigate to the "Workflows" section on the left navigation bar.

Choose the workflow scheme and navigate to the "Edit" link. Then choose transition and go to the "Post Functions" page.

Add post function to transition. Choose "Send SMS" post function and click on "Add".

Then you can choose SMS receiver and text message from custom fields values or use by default.