QoS question VoIP saturation limit?

So we use Jump desktop which is a sort of teamviewer like application to stream media remotely.

Everything works great except sometimes the users will complain about lag.

The lag usually happens when the network at the office is busy.

So I set some bandwidth rules to process the jump traffic above other types of traffic - which helped.

I guess I’m treating jump like VoIP.

So this question is for the VoIP experts.

When you guys set VoIP rules you basically give first priority to phone calls.

Say you have a 1 gig up and down connection. How many users can talk at the same time before noticing performance issues?

My point: I’m trying to understand if all of my jump traffic is set to high priority, at which point will the jump traffic itself start to cause performance issues with other jump users?

Obviously the answer is: if bandwidth is completely saturated over 1 gig with all jump traffic. But I keep seeing performance impacts when my firewall is only at %30 bandwidth.

So I’m wondering, really wondering. If QoS is setup properly, when does the VoIP traffic start to cause congestion on itself?

Like if all users do an upload at the same time, which totals the bandwidth limit of 1 gig, does all the traffic get passed at once? Or how does the pipe work?

Is it possible to saturate a 1Gig uplink with all VoIP traffic without lag, or would that much traffic cause delay in itself since it is so much?

We don’t really notice lag when it’s just a few users working. But we do notice lag when lots of activity is going on, even though the firewall is only at %30 bandwidth usage.