You would need a computer for the initial setup, and if you were to make significant changes to your account. The service is 100% web based, so you can use any computer as long as you are connected to the Internet. This includes a an Android Tablet, Chromebook PC, iPad, Linux, MacOS, or MS Windows. As long as the computer has a modern web browser and an internet connection, that is all that matters.
In addition, if you were to get a SIP phone, you would need a web browser to log into your device’s configuration panel. This will also apply to anytime you would need to make changes to the configuration (such as password change). The way you would access your device’s control panel is to go to their LAN IP address, and log in. Your device’s IP address will start with 10 or 192. This should help you discover if you are using the correct number. You would type the LAN IP address into your computer’s browser to configure the device. This will go for any SIP phone including DECT and WiFi phones.