You need to flash it with UNIStim.
You need to setup a TFTP server with a configuration file using the default file name the phone will request.
1220SIP.cfg
You might have to setup a DHCP server that can send the TFTP server address to the phone too. Hard to remember exactly but either you setup the DHCP server to tell the phone what TFTP server to use or you can manually configure the phone with a TFTP server.
For a TFTP server a nice free one is the Solarwinds TFTP server.
Put the cfg file you make and the firmware file into a folder, run the TFTP server and configure it for the folder you created or just let it create the default of c:\tftp-boot and place your files in there.
This is what the 1220SIP.cfg file will look like:
[FW]
DOWNLOAD_MODE FORCED
VERSION 062AC7J
FILENAME 062AC7J.bin
PROTOCOL TFTP
SECURITY_MODE 0
I suggest you load a version of the UNISTim firmware one version older than the version loaded in your BCM50. This will allow the phone to reboot, and then it should download the slightly newer version off your BCM50 the first time it connects. You CAN run a newer version than the one on your BCM but unless there is a specific bug fixed in a later version it's not a good idea to mix versions of BCM and UNIStim firmware that was not included in a Smartupdate since they aren't really tested together.