I assume that your callers are connected via a browser. vMix Call uses the WebRTC protocol, so the resolution is automatically regulated via the browser/protocol, depending on the bandwidth.

You can try to set the connection manually (right click on Caller in vMix, advanced settings, remote guest bandwith settings), but the better way is probably if you implement the calls via vMix to Vmix. Then the quality is much better than using a browser.

Another possibility is offered by srt. However, the port requires approval / forwarding. But with srt you can set a fixed latency.

I hope it helps you a little.