Sorry here now… I Theo (@ue-it) must have forgotten to mention me in this thread…
Don’t sell 3CX (or VoIP in general) but I do support it and I have spent a moment or two working with Peplink routers as well.
Lots of good advice here. When I do VoIP I tend to separate handsets onto their own VLAN for the sake of sanity - this then makes VoIP handset service identification easy on the Peplink (as they are in a completely different subnet when in a different VLAN).
If you are intent on keeping a single flat network one way is to do as @dennis.hofheinz suggested above and use a subnet (of your main 192.168.0.0/24 network) as the way to identify traffic.
For example a subnet of 192.168.0.64/28 would identify the IP address range of 192.168.0.65 -> 0.78 (14 host IPs you could use for VoIP handsets) which you would configure like this to use WAN2:
Or you could use 192.168.0.64/27 which would identify the hosts from 192.168.0.64->95 (30 hosts)
Another way would be to add an outbound policy for each handset MAC address - then you don’t care about subnets and IP addressing.
And finally you could identify by destination IP address or by application ports, but to do that we’d need to know whether you are cloud or on premise hosting your PBX - and I think its a pretty messy approach normally unless you’re using some sort of hosted service with a dedicated SBC/Proxy so the number of IPs is small.