I found the last registration date shown on my PAP2T was on 7/28/2009 and that's three days ago. I was using sip.voipbuster.com as the proxy and I can still ping it. But with Wireshark, I found it's not responding any packets back to the SIP Register requests from my PAP2T.
However, the voipbuster software can still log into my account and it's trying to register on another proxy: connectionserver2.voipbuster.com. Although I can't ping it, my PAP2T can now successfully register on it after I change the proxy. The server connectionserver.voipbuster.com also works.