Is this a new sim you received in the post after a upgrade?
If so you can activate the new sim by calling our customer service on 0344 809 0202 (on your landline)
You can also activate your new sim through visiting one of our stores (if possible and safe for you).
Also I can recommend that if you still have your original sim, if the sim is 4G ready and you are using another 4G device then you can use the same sim,
The only time you need to activate a new sim is if the original sim isn't 4G ready, (and is going into a 4G device) or is too big for the new device (and can't be popped down to a smaller size) or your new device is 5G ready and your original sim is 4G.