Suddenly, BSNL appears to blocking SIP ports. The clue is your device fails to register with the SIP server. For all I know this may be a regular thing because the problem keeps popping up for me once in a while in the short time I've used the device. I always thought it's a device issue but today I decided to dig deeper and here's what I found. Outgoing port 5060 (the SIP port) is definitely blocked for both tcp and udp (I test with an external BSD box where I have ssh access). So I found another VoIP provider called Callcentric which conveniently provides SIP port 5080 and SIP registration appears to go through fine.
The summary above looks simple but I had to go through a lot of trial and error. At first it appeared that ports 5061 through port 5080 were also blocked but right now just after I started posting this I was successfully able to get through to callcentric. I am using an excellent utility called sipsak to help me with debugging this. Any way, if BSNL is indeed blocking SIP calls that's really bad. VoIP is legal in India for individual users and BSNL has no right to do this. Ganesan