I'm using Switchvox, an Asterisk PBX and I'd like to host it on EC2.
Digium Switchvox provides an ISO which contains everything needed to host the pbx server: OS, software, etc. It's basically an image of the server.
How do I instantiate a new EC2 instance using the custom ISO they're providing?
From this ISO, you can create either a VMware or VirtualBox image. Form there, you may convert this image into an EC2 AMI image and go from there.
Just make sure you are using the same arch (32 v. 64) and proper kernel.
That being said, you might get into more operations then simply fire up an existing vanilla AMI available from the community. There might be one that closely match your OS requirements.