OBiAPP for PC is basically a SIP server running on your PC. It acts as a SIP-to-OBiTALK bridge, connecting SIP clients to your particular OBi. No password is used for authentication of SIP registrations. The only credential required is your OBiAPP for PC OBiTALK number. Consequently, if port 5060 can reach your PC from the Internet (due to forwarding, DMZ, no router, whatever), anyone who figures out your OBiAPP for PC OBiTALK number can register their SIP client and make calls using your OBi.