Updating firmware wrt54g
The router doesn't need to be reset to find its IP address.
Cisco/Linksys download page for WRT54G Because this router has Linux based firmware its possible to replace portions of the firmware with custom programming.
This has been verified with a Cisco 7940 running SIP 6.3 and with X-Lite.
Updating to the latest firmware will fix the problem.
If you don't know the password, reset the WRT54G router to its factory default settings, which restores the configurations back to how it was when the router was first purchased, including its password.
After the router is reset, you'll need to set up the wireless network again, plus reconfigure any other settings you had set before.
Search for updating firmware wrt54g:
Of particular interest to VOIP users, many of the replacement firmware packages add much more usable Qo S to the router than the official Cisco/Linksys firmware.