The Cisco / Linksys PAP2T lets you connect your analogue phones to VoIP services via SIP.
How to configure it: http://spakonfig.de/
Currently it costs ~40 EUR in Germany: http://geizhals.at/deutschland/a212387.html
An alternative is the Cisco SPA-3102 which also connects your phone to a classical landline connection (for failover / emergency calls).
Grandstream GXP-2000 Phone
It costs ~60 EUR in Germany http://geizhals.at/deutschland/a174281.html
A modern (and expensive) alternative is the Grandstream GXV3175 7" Touchscreen IP Multimedia Phone which costs ~ 320 USD or ~ 240 EUR.
- Free Homebrew VoIP with Google Voice and Intel Atom: http://www.legitreviews.com/article/1058/5/
- More infomation on the GXP-2000 in German: http://wiki.ip-phone-forum.de/telefone:grandstream:start#gxp-2000_enterprise_ip_phone