Incoming calls go to provider's voicemail

Hi all,
I have Free PBX running on an asterisk server. They are both up to date. I’m pretty sure I have configured my SIP trunk correctly because my provider recognizes my server. My problem is that when someone tries to call my server from an outside line, it goes straight to my provider’s voicemail. To fix this, I have to make an outgoing call on my server then, everything works fine for a while but, it will start doing the same thing again. I have no idea how to fix this so any help is appreciated. Thanks!

It sounds like your firewall is slamming the sip ports shut so that any inbound calls are rejected. if you make an outbound call, the ports probably open again. is your pbx registering with your provider or are you using ip authentication?