ZyXEL Communications ISG50-ISDN Application Note page 132

Configure the presence domain.
For example, we fill "zyxel.com.tw" in this field.
In one of the installation steps, set the country and the area code.
For example, we select "Taiwan" as the country/region and set "3" as
the area code.
If "Location has a phone system" is unchecked, all dialed digits will be
treated as "internal".
If this box is checked, the country code and the area code will be
added before the dialed numbers if more than 3 digits are entered.
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