The main page at https://www.easyvoip.com works fine, but https://www.easyvoip.com/login/ is the page they send you after you log out. It has two sets of fields: 1password fills in the username in one, and the password in the other.
1Password Version: 7.2.4 (70204000)
Extension Version: Not Provided
OS Version: macOS 10.14.2
Sync Type: Not Provided