Nope. There is no limitation of using ASTPP. You can use it anywhere.
It can be used in small to large scale carrier setup.
You just need to have system with above defined OS and then you will be able to setup ASTPP on that.
It is strongly recommended that ASTPP be deployed on the Linux distribution CentOS version 7.x or Debian version 9.x
CentOS 7.X OR Debian 9.x,
4GB RAM (8 or 16gb is highly recommended for better performance),
40gb Hard Drive,
We recommend to use high configuration hardware to get better performance.
Yeah, It can work on Virtual servers.
That purely depends on hardware which you will use. More Hardware resources can give more concurrent calls.
You can simply send your code to us for review and we will include it in the open source version.
You can donate money from here: https://www.astppbilling.org/donate-to-astpp/