We're an early stage startup [0] for web analytics and something that worked incredibly well so far is Discord. A lot of our customers/users have a Discord account and it's very easy to join a server and get into a (high quality) call, or to use the chat. That's something I didn't expect to work so well.
Appart from that, I like to ask questions in email replies and give customers the feeling to have influence on our development process (which, in fact, they have). Even if someone doesn't become a customer or signs up for a free-trial, you can learn something from them.