Draytek Vigor2500V Manual page 114

Backup Phone Number: The alternate PSTN/or ISDN number (only
Vigor2500Vi model has ISDN and can let ISDN number be backup for VoIP) to
dial if "PSTN phone number" or "ISDN phone number" is set in Loop Through
entry.
Even your home has the ISDN line and you get ISDN phone number, you
can not set the ISDN phone number into Backup Phone Number entry if
your model is Vigor2500V which has no ISDN port for connecting to your
ISDN NT1 which is installed by ISP or Telco. If you want to run ISDN
phone number as backup line for VoIP calls, you shall purchase
Vigor2500Vi series which has ISDN interface..
To manually dial the backup number
telephone handset, and then dial a PSTN phone number. If you are
worried that the automatic loop through might over charge your PSTN
phone number, we recommend you not to enter your PSTN phone
number into the "Backup Phone Number" entry. That way you can only
run loop through by manually dialing a PSTN number. For
network, the manual key shall be "#9"
Vigor2500Vi.
VoIP
via PSTN enter "#0"
as your model is
12-6
on your
ISDN