Firewall and Router

General Help
1

I have this network design.

Gateway->Firewall->Router->FreePbx

Can I put PBX like this cause this will have 2 nat.

Thank you

2

1 - Why do you need to do this? Has to be a better design.
2 - As long as the phones are on the same network as FreePBX no issues
3 - You will not be able to get Sip to the left of the Gateway