But that are how phone systems work. There are only so many lines. Once max is reach, you are lucky even to get a message. Usually the phone line will ring busy or drop.
If "there are only so many lines", then they're using decades-old phone system technology.
I ran an Asterisk server in my home when my connection was 5Mbps, and I certainly had 5+ active calls simultaneously. And with 5Mbps, I'm sure I could have done 50 without any degradation in quality.
My connection is now closer to 500Mbps, and I'm sure I could handle 5000 (actually, that might start to hit CPU limits on my server). Someone who runs a call center as a core part of their business should be able to handle 50,000 active calls without much issue. The vast majority would be on hold forever, but that's not a system limitation.