Grandstream gxw4108 Integration to Elastix [PSTN Extension to VoIP extension



I integrated PSTN to Elastix using an FXO gateway card.
After configuring the FXO gateway, i can easily call VoIP extension to any analog extension.
But incoming calls, I have faced the problem when trying to call PSTN extension to VoIP.
i call on FXO port, FXO card show there has an incoming call from an extension, but it rejects from Elastix for failed to authenticate device. How can I solve that issue?