Pretty sure government entities can, and have authorization to trace VOIP based calls if they need to. The trangulation however would depend on the VOIP providers networking method, for example, some use peer to peer and as a result they may not be able to back trace quite as easily.
But, I would consider tracing them to be similar to tracing an IP, it'll likely go to a node that your ISP has, and then they'll get your address from your account.
