Iphone 5s - Sim invalid message

[#1]

I have a fully updated Iphone 5s. When I put a plusnet sim card in the sim card port, it immediatly take me to the 'welcome' screen. When I try to restart the phone with the plus net sim still inserted im greeted with a screen message saying invalid sim. What should I do?

[#2]

Who were your original provider, when you got the phone?

It sounds like your phone might be locked to them and you'll have to contact them to get it unlocked.