The API Sandbox URL is currently timing out on me. I started noticing around 3:55pm ET.
$ curl -v https://apitest.authorize.net/xml/v1/request.api
* Trying 184.30.69.117...
* connect to 184.30.69.117 port 443 failed: Operation timed out
* Failed to connect to apitest.authorize.net port 443: Operation timed out
* Closing connection 0
curl: (7) Failed to connect to apitest.authorize.net port 443: Operation timed out
