Please ask for help before posting a negative review The vast majority of Phone Power customers enjoy excellent reliability. The fact that another trunk on your PBX is problem-free does not prove that your setup is correct, because providers differ in NAT traversal and handling of multiple registrations.
It should not be difficult to troubleshoot and fix the problem.
Besides FreePBX, are any other devices (ATA, softphone, smartphone app, etc.) registered directly to your Phone Power account? If so, have you confirmed that the inbound issue persists when only the PBX is registered? When only an ATA is registered?
Does your PBX have a public IP address? If not, please post make and model of router and modem, along with any special settings such as port forwarding that you have configured.