Sounds like someone put the system into a self-test loop. You should be able to 'end' the loopback by hanging up. If this is an on-going issue it is possible your system has been hacked, especially if you have not taken steps to protect it, like changing the default password.
If that is the case I would suggest your factory restore it, check the Admin Guide for how to do that. Once the software has been restored follow best practices for protecting that computing resouce.