On the Projects | Activity page of the calling system admin, you will see a green or red dot at the right-side edge of each campaign listed. Green campaigns are started; red ones are stopped.
To start or stop, click on the campaign row (but not on the name) to select it. The row will now be highlighted with a light green/yellow color. At the top of the page, you will see that the Start-Stop button is now enabled. Click on it to start or stop the campaign.