You have provided no information about your system configuration. I would guess that it is a server firewall/network type issue, with ports being blocked unless a call is initiated from behind whatever equipment you may have. As a first step I would suggest running the 3CX Firewall Checker and making sure that all is green.
FXO gateway so firewall should not be involved in this (inbound calls should be coming in via BT/analogue lines). However you are right that more information is definitely required most importantly:
Location of system (cloud/on prem).
Location of gateway in regards to system.
Network setup information.
Example: we host a lot of systems worldwide systems with ISDN gateways onsite (connecting to 3CX via VPN tunnel) Inbound calls come in via ISDN gateway (Patton) for DDI presentation but outbound calls go our via SIP trunks attached to the system for better call costs.
Please also advise if outbound calls are meant to be going out via FXO line also or by other means.