My library always sends me automated messages whenever something is overdue, so I'd try that?
Really I don't know. Since robocalling services charge less than 5c~ a call typically, I'd recommend buying some with a dummy message loaded if you can do so without buying in bulk. You can test it out many times differently and the fee is VERY nominal.
You can google "voice broadcasting" and probably find dozens of companies providing that service. Many of them will let you test out a few calls for free. But how in the world would this help you test your solution?