I can't upgrade Konftel 300M using the Konftel Upgrade Utility, are there any other options to upgrade?
You can upgrade your Konftel 300M with a SD card. Download the following file

Put that in the root of the SD card and then put the Konftel 300M in upgrade mode (MENU > SETTINGS > UPGRADE MODE).

Insert the SD card and press the HOLD button to read from it. Follow the instructions on the display on the Konftel.

* Please note that when upgrading using SD card it will reset the phone to factory defaults.