Voip.ms Android Subaccount Hacked Question

Hi, This happened back in January, 2017. I use Voip.ms on my android phone through Zoiper on home and public wifi, usually using IAX, but sometimes with SIP. During January 12, 2017, my android phone subaccount was accessed by someone in the states using X-Lite, making numerous calls to Montenegro, draining my account of credits. All my subaccounts use random mixed alphanumeric and special character passwords generated through Lastpass. Just asking if anyone knows how this could of happened. Thanks, Geod