New iPhone 5 has no service?

I got an iphone 5c, but I didn't realise it needed a nano sim so I had to cut my micro sim, I put it in and it worked fine but it says there's no service, I've changed all the service settings and I get signal for a few seconds but then it goes. And every time I try to call someone, it goes straight to saying call ended.

Get new sim

It may be that the SIM is not in exactly the right location & shorting out or not making proper contact.

Are all the SIM contact pads completely within the cutout of the SIM tray? Is it a good enough fit that it can't move and the contacts touch the metal of the tray?

If it's a poor fit or the pads are not central in the tray, don't risk trying to wedge or pack it with anything, there's too much risk of damaging the phone if any tiny bits get loose inside it.

Contact your phone provider and ask them to send you a micro SIM. Most do it free of charge, and most have an online SIM swap page that allows you to activate the new SIM and move your number to it in a matter of minutes.