ZyXEL Communications ISG50-ISDN Application Note page 91

Single Number Reach (IP phone & Mobile SIP & GSM Mobile Number)
Go to CONFIGURATION > PBX > Extension Management > Authority Group > [Group Name] > [Extension Number] > Edit > Call Forward to
Configure your mobile extension settings.
You can select to "Manually" turn on and off this
feature or select "Force Enable" to always turn on this
feature.
Fill in a mobile phone number or an extension number
and select a dial rule (LCR) from the list.
In this example, ISG50 rings the extension 1007 and the
mobile phone number 0912345678 simultaneously
with a single number.
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