Hello John,
From the various forums out there, it looks like your unit is a VOIP product from Vonage. You may need to get in touch with your VOIP provider for the firmware as it looks like the firmware is specific to Vonage or other 3rd party VOIP providers.
Here is a link to the Linksys site, but there is no firmware listed.
http://homesupport.cisco.com/en-us/wireless/lbc/WRTP54G
I could not find any links for firmware download, but found this thread.
http://homecommunity.cisco.com/t5/VoIP-Routers/WRTP54G-firmware-upgrade-user-pass/m-p/3609
Hope this helps somewhat.
Bert