I had a problem with registering a device(in this case a Mitel 6920) which gives each time a status code 606.
So the device could register with the username and password, but there was something else wrong.
After spitting the internet and doing a lot of other stuff i finally looked to the firmware of the device.
Unlike the other Mitel devices we are using, this device wasn't updated to the latest firmware version we are using for Mitel devices on the PBX. After updating to the (newer) firmware, the device registered perfectly without status code 606.