This is actually one of the reasons why I ordered a new Note 9 to replace my current Note 4 Edge (this and horrible reception during calls)
I really hope this isn’t an issue with the new phone
Go into Settings in the phone dialer and make sure the option to forward calls to voicemail is not turned on
It's not any obvious thing like that, there are no settings making this happen, it's a fundamental bug with the note 9.
This worked perfectly on my wife's Note 9 with T-Mobile. Go to Call Settings -> More Settings -> Call Forwarding. Make sure that the first option, "Always forward" shows "Turned off".