I had the same problem so I downloaded the firmware from the Polycom download matrix and upgraded manually. If you do this download the combined load which supports older BootRoms. Get a free tftp server and configure its base to the extracted SIP load and point the phone to your computer. If you’re on a laptop or workstation you may have to drop your firewall, or open port 69. The phone will upgrade automatically and you should be able to point it back to the pbx after it’s done. It may have to reboot a couple of times. Read the instructions which provide alternate upgrade solutions for old boot loaders where they supply a link to the upgraded BootROM.