I actually screwed that up, we only have 3 total software engineers. Including myself.
We do have other employees who maintain the hardware, on-call DBAs to manage issues, etc. I'm only speaking to the software engineers.
And we have lots of hardware and lots of open-source solutions that handle the actual calls.
I'm full-stack but lead on the front-end, a complex Angular application to manage everything from huge call centers to small restaurants.
We have C# for APIs, cloud Oracle for the database, and a whole slew of other software and services to manage the actual calls.
Each of us is specialized in specific parts. Our up-time is tremendous given the amount of code we've written. It's extremely stable.
I've been at this 20+ years, as have the other 2. We know enough between us to get this done.
I've never been called after normal work hours. We release the updated front-end every week and haven't had any issues. And a lot of changes/improvements go into that ... that's a lot of my job.
It's well-architectured, well-tested, fault-tolerant software.