SIP

SIP Firmware can be downloaded direct from the Mitel MiVO 400 FW Page
See Contents Here
Path to use for phones will be

https://fwdl-mivo400.mitel.com/6700i/7-2

The phone will automatically request the firmware based on the model, e.g 6920.st etc...

Can also try the below for version 6.6.0.0.60 (R6.6.0 GA)

https://ns8.lab.hexicor.cloud/sip/latest/