How to separate traffic?

Question

openvpn

Good day. I'm posting my stupid questions again as the tasks get more difficult.
Let me describe the situation first.
There are two offices. Each office has a Debian-based gateway, with two Internet channels, and, accordingly, with 3 network interfaces - two for Internet, one for LAN. Internet channels have different GWs and subnets, for example 94.180.zzz.180 with GW 94.180.zzz.255, and 94.180.xxx.235 with GW 94.180.xxx.255. The gateways are connected to each other via OpenVPN. On the client side of OpenVPN, there is a telephony server inside the network that uses this tunnel. So here's how to wrap either only telephony traffic, or all tunnel traffic on one channel, and the rest, "external", on the second? I tried to wrap the tunnel route on one of the channels, I ran into the problem of asymmetry.
If not difficult, explain on the fingers or with an example.