Well, if BSNL's VoIP server sees traffic from an internal private IP address how is it going to send the return traffic back to you? It wouldn't know how to send it back cause yours would be from an unroutable IP address or atleast BSNL wouldn't be aware of the route.
So you definitely need NAT, and any traffic going to the VoIP server needs to use the 10.x.x.x IP address allotted to you as the source.
I definitely got it working with a static route so it's something in your configuration.