For your phoners, you will need computers (Windows, Chromebooks, or Macs) or tablets - as long as they can run a current version of software, it should be fine.
Phoners will need either a headset to plug in to their computer or a phone (cell phone or landline) to handle the audio.
For one or two phoners at a single location, any modern internet connection should have plenty of bandwidth.  A stable connection is the most important factor.
If you are setting up a phonebank at a central location, each computer will need approximately 125 kb of bandwidth, both upload and download.  This means that each megabyte of bandwidth is enough for 8 phoners.
For the people in charge of running the phonebank, you will need at least one Windows-based computer.
The calling system runs in a browser. Chrome generally works for most people; other browsers work well, too.