Error while testing Donation API

I'm working on adding donation to support to the PHP API library I use for interacting with Convio, and I'm getting an error that reads:

Declined due to Convio IP Block

as the reason it declines the credit card provided (I'm sending the fake card number provided in the documentation).

I can't find error responses and reason documented anywhere. Can someone explain this error? I thought it might be related to the IP address of the server, but all my calls to the Constituent API from that same machine work just fine.

While we're at it is there documentation out there on error codes? I'd like to have a complete listing so I can work out proper handling. It's one thing to fly blind when adding a constituent to a mailing list, but the donate command can throw so many more errors that it seems like trial and error is a bad way to find them.

Aaron