End of last year I bought a Siemens IP Phone and I am very happy with it, already thought about buying a second haedset handset (to expensive though). Anyway, there is new version of this Gigaset IP Phone, check this review (in German) and this page called "gigaset.net"

update: actually I have to update my firmware to test out gigaset.net, here we are:
Gigaset C450/460 IP (V01063)
Download version: V01063
Firmware Update 6th of July 2007 Version V01063
Improvements:
* OPCAP compatibility improved.
* Dutch Net Directories Country Code problem solved.
* Minor changes in the Web Configurator conducted.
* Dutch Clip - GAP problem solved.
* Polish language added.
* Gigaset.net phonebook problem solved.
* G.729 Audio Codec speech quality improved.
* German provider Dokom compatibility improved.
* Turkish provider Dogan Telekom compatibility improved.
You don't need to download files. The firmware is updated directly online by the Gigaset C450 IP. You can easily initiate the update by three steps:
1. Menu Settings » Base » Firmware Update
2. Enter base station system PIN (default setting: 0000)
3. Press the display key "Yes" to start the download
If your firmware is already up to date, the handset informs you. No action is needed then. sounds easy, doesn't it?
update vom 17.08.
Da ich mir nicht sicher war, ob mein Telefonbuch am Handset und die VOIP Einstellungen im Webclient erhalten bleiben, habe ich (bis jetzt) auf das Firmware update verzichtet. Schliesslich funktioniert das Telefon wunderbar und ein gigaset.net Gesprächspartner habe ich nicht.