The future has just begun, our phone and DSL connections were switched over from the big pink monopoly to Arcor. Unfortunately, their phone rates border on highway robbery (by being at least double as expensive as cheaper providers). So, I decided to give Voice-over-IP a try, using kphone as my SIP client. These configuration instructions were sufficient, except for one minor (but important) detail: You may not use Umlaut characters as part of the Name field in KPhone, otherwise the sipgate server will report back something like (see the KPhone console output)

SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP 82.82.79.89;branch=RANDOM-CHARACTERS;rport=5060
CSeq: 1722 REGISTER
To: "Andreas Klöckner" <sip:5557868@sipgate.de>;tag=--------------HEX-DIGITS-------.HEX
From: "Andreas Klöckner" <sip:5557868@sipgate.de>
Call-ID: NNNNNNNN@192.168.113.4
WWW-Authenticate: Digest realm="sipgate.de", nonce="------HEX HEX HEX HEX------"
Server: sipgate ser
Content-Length: 0

Hope this helps someone out there.