Sorry to bump this, but as I was editing a SipBroker Alias this came to mind:
-Go here
SIPBroker - EziDial
-Enter your VoXalot SIP URI (or *010xxxxxx) and you'll receive a call that connects you to the echo test (not really a callback, but close)
-This is also neat if you want a quick way of testing that certain providers are accepting calls via SIP URI and what the voice quality/delay is like