Gigaset SX553 WLAN dsl First Steps page 95

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In the Service provider menu, select the option Other or else select one of the
already preconfigured providers.
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Enter the data you have received from your service provider:
User name, Displayed name, Authorization user name, Password, SIP domain,
SIP realm, Proxy server address and Registrar server address.
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Leave the default settings for the parameters SIP listen port, Proxy server port,
Registrar server port, Voice codecs and Out-of-band DTMF unless your service
provider has provided you with other data.
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Click the OK button to apply the settings.
Configuring the Advanced Settings
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