Draytek Vigor2830 Series User Manual page 258

For Block Unknown Domain – this function can block incoming calls (through Phone port)
from unrecognized domain that is not specified in SIP accounts. Such control also can be done
based on preconfigured schedules.
For Block IP Address – this function can block incoming calls (through Phone port) coming
from IP address. Such control also can be done based on preconfigured schedules.
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Vigor2830 Series User's Guide