Why won't my new Iphone 5 sim work?

My employer went bust and my work's phone got cut off, but no-one ever asked for it back. So I figured that I could get a sim only deal on Orange (same network as before) and keep using it. But I've picked up the sim today and it won't work. Any ideas, or have I just wasted my money (albeit only a rolling monthly contract)?

Is the sim activated yet?

Did you connect to itunes and activate service with the new sim card?

you just can't sim swap on iphones like you can on androids, windows, and blackberries… Apple is communistic… You'll learn

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