I'm guessing this isn't installable on 2007HE?
edit: Couldn't wait for an answer so I went ahead and tried. Here's my notes from a sucessful install on OS 2007HE on a 770!

Went to site, did one-click install. Application Manager came up and tried unsucessfully to install it.
I checked what binary /usr/share/applications/hildon/osso-rtcom-installer.desktop was referencing: /usr/bin/betainstaller. So i got root and ran that command. Poof the App Manager opens again, does some sort of downloading or...? And then says rtcom is already installed. Hmm. So I cautiously reboot...and there's new options available in Internet Call. I successfully set up my gizmo project account as per directions on the rtcomm site.

..and made a test call to a nearby payphone Can't say how stable/usable it is just yet, but so far so good.

Here's what seems to be a list of what /usr/bin/betainstaller installed:

Nokia-770-36:~# ls -tla /var/lib/dpkg/info | more
drwxr-xr-x 6 root root 0 Oct 5 13:28 ..
drwxr-xr-x 2 root root 0 Oct 5 13:27 .
-rw-r--r-- 1 root root 157 Oct 5 13:27 osso-rtcom-beta.list
-rw-r--r-- 1 root root 336 Oct 5 13:27 osso-speeddial.list
-rw-r--r-- 1 root root 438 Oct 5 13:27 telepathy-stream-engine.list
-rw-r--r-- 1 root root 598 Oct 5 13:27 telepathy-sofiasip.list
-rw-r--r-- 1 root root 436 Oct 5 13:27 osso-statusbar-presence.list
-rw-r--r-- 1 root root 658 Oct 5 13:27 osso-voip-ui.list
-rw-r--r-- 1 root root 683 Oct 5 13:27 osso-mission-control.list
-rw-r--r-- 1 root root 673 Oct 5 13:27 osso-chat-ui.list
-rw-r--r-- 1 root root 1546 Oct 5 13:27 osso-addressbook.list
-rw-r--r-- 1 root root 528 Oct 5 13:27 osso-applet-presence.list
-rw-r--r-- 1 root root 210 Oct 5 13:27 telepathy-feed.list
-rw-r--r-- 1 root root 462 Oct 5 13:27 telepathy-gabble.list
-rw-r--r-- 1 root root 671 Oct 5 13:27 osso-accounts-plugin-sip.list
-rw-r--r-- 1 root root 819 Oct 5 13:27 osso-accounts-plugin-jabber.list
-rw-r--r-- 1 root root 181 Oct 5 13:27 mission-control-filters.list
-rw-r--r-- 1 root root 812 Oct 5 13:27 osso-accounts-plugin-gtalk.list
-rw-r--r-- 1 root root 285 Oct 5 13:27 libtelepathy-glib0.list
-rw-r--r-- 1 root root 502 Oct 5 13:27 mce.list
-rw-r--r-- 1 root root 294 Oct 5 13:27 mcetools.list
-rw-r--r-- 1 root root 809 Oct 5 13:27 mime-support.list
-rw-r--r-- 1 root root 349 Oct 5 13:27 libsofia-sip-ua0.list
-rw-r--r-- 1 root root 344 Oct 5 13:27 libscw.list
-rw-r--r-- 1 root root 389 Oct 5 13:27 libsofia-sip-ua-glib3.list
-rw-r--r-- 1 root root 299 Oct 5 13:27 libosso-rtcom.list
-rw-r--r-- 1 root root 309 Oct 5 13:27 libmissioncontrol-utils.list
-rw-r--r-- 1 root root 388 Oct 5 13:27 libloudmouth1-0.list
-rw-r--r-- 1 root root 290 Oct 5 13:27 libimlogger0.list
-rw-r--r-- 1 root root 288 Oct 5 13:27 libsqlite3-0.list
-rw-r--r-- 1 root root 421 Oct 5 13:27 libaccounts-rtcom0.list
-rw-r--r-- 1 root root 355 Oct 5 13:27 libfarsight-0.1-2.list
-rw-r--r-- 1 root root 433 Oct 5 13:27 libglade2-0.list
-rw-r--r-- 1 root root 674 Oct 5 13:27 gst-plugins-farsight.list
-rw-r--r-- 1 root root 454 Oct 5 13:27 farsight0.1-2-transmitter-rawudp.list
-rw-r--r-- 1 root root 472 Oct 5 13:27 farsight0.1-2-transmitter-libjingle.list
-rw-r--r-- 1 root root 448 Oct 5 13:27 libjinglep2p0.list
-rw-r--r-- 1 root root 379 Oct 5 13:27 libjinglebase0.list
-rw-r--r-- 1 root root 407 Oct 5 13:27 farsight0.1-2-rtp.list
-rw-r--r-- 1 root root 404 Oct 5 13:27 farsight0.1-rtp.list
-rw-r--r-- 1 root root 352 Oct 5 13:27 eds-sync.list
-rw-r--r-- 1 root root 627 Oct 5 13:27 evolution-data-server-addressbook.list
-rw-r--r-- 1 root root 228 Oct 5 13:27 libosso-abook.list
-rw-r--r-- 1 root root 616 Oct 5 13:27 osso-contact-plugin.list
-rw-r--r-- 1 root root 487 Oct 5 13:27 galago-daemon.list
-rw-r--r-- 1 root root 242 Oct 5 13:27 libgalago3.list
-rw-r--r-- 1 root root 496 Oct 5 13:27 libmissioncontrol0.list
-rw-r--r-- 1 root root 286 Oct 5 13:27 libtelepathy.list
-rw-r--r-- 1 root root 175 Oct 5 13:27 evolution-data-server.list
-rw-r--r-- 1 root root 157 Oct 5 13:27 mission-control.list
-rw-r--r-- 1 root root 444 Oct 5 13:27 osso-applet-accounts.list
-rw-r--r-- 1 root root 1507 Oct 5 13:27 osso-rtcom-icons.list
-rw-r--r-- 1 root root 252 Oct 5 13:27 libaccounts0.list
-rw-r--r-- 1 root root 424 Oct 5 13:27 libebook.list
-rw-r--r-- 1 root root 163 Oct 5 13:27 libmissioncontrol.list
-rw-r--r-- 1 root root 341 Oct 5 13:27 libedata-book.list
-rw-r--r-- 1 root root 349 Oct 5 13:27 libedataserver.list

AWESOME! I hope this proves more stable than Gizmo's fuxored client.

double edit: As it says on the garage page, "DTMF is not reliable." ...meaning I can't check gizmo voicemail via their 611 number. Other than that, it's working excellent. Receiving and making calls is a bit quicker compared to the 10 second 100% cpu usage gizmo goes through. Tested and nicely functional with http://click2voice.com

..and that buttugly default icon (some sort of flat penguin?) is located at /usr/share/icons/hicolor/128x128/hildon/qgn_addr_icon_avatar.png for your haxing pleasure..
you can also change the heinous little purple dudes with headsets on to something else. It's at /usr/share/icons/hicolor/scalable/hildon/qgn_indi_voip_idle.png