http://vultr.com allows you to install from ISO.
You don't need to install with the ISO. There is an install script that will put it on any vanilla Centos 7 install. Better yet, use the install script as a guide and install manually. You will learn a lot which will come in handy when you inevitably need to troubleshoot something, and I can guarantee you that will happen.
I don't think Amazon AWS is ideal because I think they use something like NAT which makes life more complicated for VoIP. A provider like Digital Ocean or Vultr is going to be less expensive and no NAT issues.