The Sip call drops after 30 seconds, but it doesn't always happen. I think it's because of NAT timeout. Am I correct? Or is it something else?
It's because 30 seconds is the timeout value for SIP transactions and it's probable that the ACK request, which completes a call INVITE transaction, is not getting through.
As to why the ACK request is not getting through there are a number of possibilities but it's unlikely to be NAT. If it was a NAT issue then the initial INVITE request is unlikely to have reached the callee SIP device.
A common issue can be SIP Application Layer Gateways (ALG) built into home routers. They will often apply very crude text replacements on private IP addresses in SIP packets and this can be enough to break SIP transaction matching logic. Check the specifications on your router model and if it has a SIP ALG turn it off.