You can work on voter registration. There’s got to be some potential new voters who were in the ‘None of the above’ category in 2016. Also, a lot of Bernie 2016 voters were college students who have graduated and moved states (or at least cities) and need to register in their new area. I think it’s safe to say that Trump really hasn’t grown his voting base and it skews older and more stable, so I doubt you’ll be registering too many new Republicans.
At least most of my phone banking has been to contact people who have filled out a card or web form asking for more information, that’s the preferred method when you’re working a table at an event, that way you won’t tie up a volunteer for an hour when you’ve only got three hours at a farmers market or something.
Oh, and a lot of it is done by texting now, easier to reach more people and convenient for their schedules.
I basically went to HRC’s and state-level Dems websites and signed up to volunteer for various things. They usually want people for door to door canvassing, but they’ll certainly take phone banking if that’s what you’re comfortable with.
As Dale said, they give you the lists of numbers to call, addresses to visit, and those lists contain the names of people who’ve contacted them at some point in the past. Be aware that affiliations can change, and people can change addresses LOL! I had a Trump supporter tell me to 'GTFO (his) porch and that Hillary ‘oughtta be thrown in fucking jail.’
Oh yeah, phone banks are one thing, but you need a thick skin to go door to door.