I picked up a Vertical Communications 7504-10 IP phone from a yard sale a little while ago on the assumption that since it was standards-based (SIP) that I'd be able to use it with Asterisk. Browsing around with google only revealed one thread where someone claimed that you could make the phones work, but said nothing about getting into the phone in the first place. But as it turns out, it's actually quite easy to configure these phones to work with an arbitrary SIP provider.
I came across a relevant thread from the asterisk-users mailing list way back in 2010 these phones are discussed. They're available for around twenty-five dollars and up on eBay. The answer lies in this application note on defaulting Xcelerator IP and IP2007 phones. The short, short form is that the default admin password is 1234, and you can reset the phone to factory defaults by booting the phone and then holding down the reset button (next to the network status LEDs) until the unit reboots. It will make a tone to let you know it's doing this, so you don't even have to watch the display.
Once you've reset the phone, you can get into the admin menu. The first of four buttons below the display should be the config button, as shown on the display. Then you can get into the admin menu with the password 1234. Once there you can configure your network type (probably to DHCP) and your SIP account information. The only hassle is having to use a phone keypad to enter passwords. For testing purposes, you will want to make them short and numeric. Once you have things up and running you can go on to the next phase.
The "left" button on the D-Pad is "back", while the check-mark button in the middle of the D-Pad is enter. And this is the real kicker: You need to exit the menu in order to save your settings. You can't just make some settings and then use the reboot function in the admin menu. You have to change the settings, exit the menu at which point the phone will save settings and tell you about it, then go back into the menu in order to reboot with the new configuration. This hint should save you at least one phone reboot, which is not particularly rapid.
Another secret you may or may not need to know is that holding down the first and fourth buttons below the screen during boot will bring up some kind of tftp menu at least some of the time. Upgrade? Config? Not sure.
Either way, you can either configure the phone through the menus, or through the web interface, at least once you've got the network up. For asterisk on your local network, there are few steps. Configure the network; I used DHCP. Set the proxy server, outbound proxy server, and registrar server to the asterisk server in the SIP settings, the user name and the authorized ID to the asterisk user name, and the time zone in the system settings. There's also a voicemail number which you might reasonably want to set to whatever your dialplan specifies for fetching your voicemail. These settings are enough to get the device up and registered with asterisk while, for example, following the Beginning Asterisk guide. Then you might reasonably use Linphone for your other extension (ekiga crashed on me when I tried to use it...)