on 09-11-2016 23:52
Hi @Anonymous You need to activate the sim...Follow the advice in this link.
Plenty of different scenarios and one of them should work for you.
If this is a new replacement sim (due to you losing the original) then Customer services will need to activate it
Best of luck
on 10-11-2016 06:25
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)