Sadly, the ping service on the VoIP.ms servers HAS BEEN TURNED OFF. This was done as "a matter policy" according to the developer I spoke with. This means that the "Server Ping Tests" feature of my app will have to be removed, as I no longer have a way of testing ping times.
I thought of just timing how long it took to get a response from an API call, but these don't reflect pure latency, and include any delays introduced by the server when responding to a request. If anyone knows of an alternative that works, I'd be happy to give it a try.