This error appear because your gateway IP not at the same subnet of 3CX box. It should be the bug of "Generate config file". The work around is change the gateway IP(Dummy) to same subnet of 3cx box > Generate config file > download to local drive. Then change back the real gateway at 3CX box. The downloaded config file what you need to do is, use text editor open it search the dummy ip then replace with the real gateway. Save it then import to your Patton gateway.Unknowing device host.Impossible to resolve subnet mask
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