How can I bookmark certain calls to go back to?

On the Calls page you can click the ellipses to select Listen Later under a particular call. On the individual call page, there is just a button at the top left of the page to click that says Listen Later. Once you have selected a call to listen to later, you will see the calls listed on your Home Screen under To Do. You can also add a call to your library (just a reference, doesn't impact others) and then go and find the call in your library and listen to it from there later.