NEC SP5008A - Manual (Page 50)

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FXO Dial Prefix For outgoing call, the default will be made to VOIP.
However, if you set the FXO prefix here, the call will be
route to FXO instead of VOIP. It can be applied to FXO
model only.
1. Press the “Apply” button (at the bottom) after you finish to save changes.
2. Press the “Reboot” button to apply the changes.