You will be able to use Sub Sidekick without a smartphone but services will be somewhat limited.
With a smartphone, you would be able to click on the link to view the job alert and then either accept/decline the job from your phone. If you do not have a smartphone, you will still be able to receive text alerts on your phone, but will simply need to log-in to your substitute-service's website after the alert in order to actually accept the job. If your substitute service provides a call-in number, you may also call to accept the job. Even though not having a smartphone keeps you from being able to accept the job on your phone, receiving our alerts will still give you an advantage because it informs you of jobs even when you are not able to be at your computer at that moment.