Are you looking for firmware to upgrade the phone or just convert it to SIP?
As far as I know and have experienced, the 5320 phones are dual mode so there is a standard SIP mode built in.
1. Disconnect the power from the set.
2. Hold down the "*Superkey*" while powering up the set.
3. The set will come up and ask if you want to CONFIGURE PHONE? select YES.
4. "NETWORK PARAMETERS?" select NO
5. "HARDWARE CONFIG?" select NO
6. "PHONE MODE?" select YES
7. "PROTOCOL?" select YES
8. "PHONE MODE: Minet" select Change
9. Select "SIP", "Accept" and confirm with "Yes". It will now save the settings to NVram
10. "REBOOT NOW?" select Yes