If you know (or suspect) the network you are on has a firewall, go ahead and try to connect the two units. The firewall may already be properly configured. If it is not, then the following things should be considered.
If a RoIP server unit is behind a firewall, then the server unit’s VoIP port needs to be opened on the firewall for both TCP and UDP. This is called “opening a hole in the firewall” and should only be done by (or with the cooperation of) the network administrator. This will allow TCP and UDP packets through the firewall so they can be received by the RoIP server unit.
If a RoIP client unit is behind a firewall, typically nothing needs to be configured on the firewall. Most firewalls will allow traffic back through them that is related to a connection that was initiated by a device on its network.