Incoming call not always in

I have one small setup and incoming call is strange, and it only happen sometime.
When I dial the DID, some time ( it only happen some time ) I will get “the call can not completed please check number and dial again …”, then I hang up, after few seconds, I dial again, the call will go through.
outgoing call is fine.

we have internet connection with static IP address, but in sip setting, I still pick up dynamic, and use no-ip setup dns record for getting the IP.
I tried public IP and static IP option, and they are not working at all.

our sip privoider only provide limited info as credential for authnenticion, host address, and port need to be opened.

I am new to FreePBX, don’t get what need to be done during troubleshooting procedure .

Thank for any help.