setting up sim on ipad air 2

Hello and welcome to Apple Support Communities, Dewi208.

If I understand your post correctly, your iPad and iPhone have different phone numbers but when you send a text message from your iPad it appears to have been sent from your iPhone. I’d like to help.

It sounds like your iPad and iPhone have text forwarding set up. To turn this off, see this guide:

Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch

Let us know if that resolves this issue for you. 

Thanks for using the Apple Support Communities. Take care.

Set up SIM card for what exactly? Your iPad can not receive text messages (or phone calls) like a smartphone.

That's correct. The SIM card in iPad is used for data connection only. You will be able to receive text messages and voice calls if you pair the iPad and iPhone. See CarlosP1228 reply for more info.