In this scenario a static route like you are proposing is pretty much pointless as it effectively already exists. ClearOS knows that any traffic arriving at enp4s0 for any subnet other than 192.168.3.0/24 will automatically get routed to enp2s0 while ClearOS is in gateway mode.
Are you running any IDS/IPS or other filter on ClearOS?
Looking at https://www.3cx.com/docs/manual/firewall-router-configuration/, their examples are really for configurations where the softphone in on the internet and there is nothing indicating that you need to forward anything so I am puzzled. Have you tried asking 3CX?
I wonder if you can turn off NAT for traffic for these phones by doing something like:This is only a guess. If it works, it means the 3CX server will receive traffic which appears to come from 192.168.3.0/24, rather than being natted to 192.168.2.2.
If you can't get any help from 3CX you may need to start packet sniffing but I am not sure how to diagnose the issue.