Is the headset plug in 2.5mm? If not, what is the size?
Is the headset plug in 2.5mm? If not, what is the size?
No. It connects using desk telephone style "registered jack" plugs, the 4P4C plug shown in this Wikipedia article:
https://en.wikipedia.org/wiki/Registered_jack
It is not for use with PCs, laptops, mobiles, or cell phones.