Test My SIP sends out a SIP OPTIONS message and displays the response.
This is a great way to confirm that the SIP port is open and the SIP device is responding to SIP messages.
From the SIP RFC:
The SIP method OPTIONS allows a UA to query another UA or a proxy server as to its capabilities.
This allows a client to discover information about the supported methods, content types, extensions, codecs, etc. without "ringing" the other party.
For example, before a client inserts a Require header field into an INVITE listing an option that it is not certain the destination UAS supports, the client can query the destination UAS with an OPTIONS to see if this option is returned in a Supported header field.
All UAs MUST support the OPTIONS method.