How to get new SIM ONLINE that fits new phone same number

[lnrgms]

I want a SIM that fits my new smartphone but to keep the same number of my old brick phone. How do I do this ONLINE? I went to the Swap SIM page but it didn't ask me what type of phone I had so I think I did the wrong thing. Why is such a basic thing so hidden in the website? Where it says "my device" there should be an option "change device".

[MI5]

@lnrgms 

You'll need to call customer service, go in store or order any free sim from here https://www.o2.co.uk/shop/sim-cards/pay-as-you-go.

Once you have a new sim, you then fo a sim swap Guide: Sim Swap: a mini guide 2017 update 
Numbers here to call O2 Guide: Coronavirus Community Help and Support 
Or advice from an O2 Guru regarding a way to get through:
If you call 4445, when it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.

[lnrgms]

Thank you for your reply.

On the page you linked I see lots of different bundle options, what do they mean if it's a free SIM I'm ordering?

[MI5]

@lnrgms 

As I said, just order any of them.

It doesn't matter as you will do a sim swap and transfer all your current sim details to it anyway.

[Bambino]

@lnrgms Even if your phone is old your current sim card may have perforations. Check it to see if you can pop out the correct size that you need.

[lnrgms]

@Bambino No, it only goes down one size (to micro) and it's too big. My new phone needs a nano.

[Bambino]

It was worth a try, @lnrgms. In that case you'll have to call or go in store, as previously suggested.

Edit: Going in store is the quickest option, as they will swap your sim for you while you're there and you'll be back up and running usually within the hour.