The only way that your admin password can be changed, apart from by yourself, is if it has been changed by someone who has access (with or without your permission) to your router's Wi-Fi or Ethernet. Not knowing which package you have, I cannot yet comment on your VoIP configuration in your Sagemcom Hub 2 (5464).
If you reset the configuration, unless the router is faulty in some way, which is unlikely, it will revert to the password on the rear of the router. Once done, I would also recommend changing the wireless password to a stronger one as well, in case this has been done by someone who should have access to your router.
