Underpass is built for total privacy. It does not collect any information, and it does not have any analytics. Underpass does not request your phone number, email, contacts, or any other personal identification. You don't even need to use your real name.
Underpass never "phones home". It is peer-to-peer, so it makes a direct network connection to another Mac or iOS device running Underpass. Your chats are completely independent of the developer of Underpass and never go through the developer's computers.
There are links in Underpass Help to contact the developer if you need support, but those links only open if you choose to click them yourself. If you contact the developer for support, your email address and other information will be kept private and never be shared with anyone.