U Chat Messenger use your phone number because that is how UCM routes chat messages between you and your contacts, similar to how the SMS system would.
In order to prevent and reduce unsolicited spam messages, as well as to improve your in-app experience, UCM uses the phone numbers from your phone's address book. UCM does not store any of your information on anyone else's systems. UCMlooks at the phone numbers in your address book and then checks to see which of those numbers are verified in UCM. This allows any UCM users from your address book to appear as contacts in your Favorites list (Contacts tab on Android) and the Chats screen. During this entire process, only phone numbers are sent to UCM for lookup, securely, over an encrypted connection. No other information, such as names, emails, and addresses are sent. So that you know who you are chatting with, the app then displays the names from your address book.