When a user has forwarding active, any call routing, including barring for calls to that user, will be applied. If a user cannot make long distance call, and attempts to forward to a long distance number, the call will fail. As call routing/barring can vary by time of day it is not possible to block the attempt to configure long distance as the forwarding target.

Use the setting System > Telephony > Telephony > Inhibit Off-Switch Forward/Transfer to inhibit all off-switch forwarding and transfers. When enabled, this takes precedence over all user settings.

This setting can also be set per user using User > Telephony > Supervisor Settings > Inhibit Off-Switch Forward/Transfer.