This server could not prove that it is prepaidselfserve.koodomobile.com; its security certificate is from incapsula.com. This may be caused by a misconfiguration or an attacker intercepting your connection.
It will then ask you if you still want to proceed or not. I clicked proceed, and nothing bad happened. It simply just loaded the website.However, in the very slim chance that the website was hacked, I'd still enter my account details to check my usage and stuff- just don't enter any credit card information or personal info.