Is Telstra, Optus, Virgin, Vodafone, Three, Orange or any other overseas telephone company represented in PNG?
No and there are only limited global roaming agreements with the local telcommunication provider Telikom PNG
The only two that I am aware of are Post Paid Telstra and Optus SIM's. These should allow global roaming while in PNG. Digicel also have some agreements with other telephone companies outside PNG.