How do I configure my Zoom ADSL X6v modem/router for 1und1 VoIP service?

First, you must log into the Zoom Configuration Manager and access the VoIP subsystem:

Open your web browser and type http://192.168.0.1 in the address bar field. Press Enter.
When the authentication dialog opens, type the following user name and password in lowercase letters, and then click OK.
User Name: admin
Password: zoomadsl (where "l" is a lowercase letter L)
When the ADSL Setup page appears, click the VoIP icon on the Zoom menu bar.
Click the Advanced VoIP Setup icon on the Zoom menu bar.
On the left pane, click VoIP Accounts.

On the My VoIP Account page, enter the following:

Parameter	Setting
Turn My VoIP Service	On
My VoIP Provider's Name	1und1
My Caller ID When I Call Someone	Type your name or a display name you want to use
My VoIP Phone Number (SIP User ID)	Type your 1und1 number (e.g., 4912341234)
My VoIP Service Authorization ID	Type your 1und1 number (e.g., 4912341234)
My VoIP Service Authorization Password	Type your 1und1 password
SIP Server	sip.1und1.de
Auth Domain	Leave this field blank
Outbound Proxy	sip.1und1.de
Register Domain	sip.1und1.de
ReReg Interval	120
Subscribe Domain	Leave this field blank
ReSub Interval	1800
Use Outbound Proxy for REFER	Disable
Ring Type	1
Dial Prefix	Leave this field blank
Preferred Codecs	Leave these settings blank

Now click Save These Settings and then click Reboot VoIP.

For a screenshot of an example configuration, click ATA_1und1.jpg in the File Attachments area below.