Your credit rating is impacted and you could end up with default. Source : I did this for someone at the time.
The person I helped couldn't get a phone for various reasons (needed a deposit, didn't pass checks, didn't bring ID) we ended up going to the 02 shop where the salesman told my friend the only way they could get accepted was if they knew someone on 02 (cue eye glance towards me)
Daft me agrees under the proviso that although my name was on the account, my bank details wouldn't be used (thought I was being smart) sales assistant reassures me that nothing would happen to me blah blah blah.
3 months later I get two emails, one for my bill, one for friend. Friend had ran up a bill of over £600. Bill was itemised and gone over their minutes allowance and whatnot. Friend was on JSA so I knew they couldn't pay. We had to contact 02 and friend went onto a payment plan. I thought nothing of it as I said earlier, I didn't have to pay, it wasn't my bank account and this was 2011.
Friend didn't stick to payment plan and ultimately debt went to another agency. With fees the £600 went up to £1000. They said if I paid in full I only had to pay around £450. I had the cash but was fuming. I agreed to have it paid with a written letter that it was closed. And I was still unaware of the default. Friend did pay me back in instalments.
Cue 2015, for some reason I decide to check my credit score, I see I've got a fucking default on my record.
So in short. DO NOT DO IT. It's not worth it. There's a reason they have these checks in place. Don't be like me.