Here's how the offers generally work:

1. You order the phone (tip: Try to get a contract which includes all the inclusive minutes and texts that you need - The retailer will not reimburse you for extra calls and texts that you make when you have used your inclusive minutes and texts up).
2. Use the phone as you normally would, you receive the phone bills, which you initially have to pay .
3. You claim back the line rental you have paid out from the retailer (often in two, three or four stages during the contract).
4. After the minimum contract term is over, you sign up for another free line rental deal and cancel your existing contract.

To help you to understand what it is you have to do, here are some links to the some of the retailers Terms and Conditions (which include their cashback instructions somewhere on the page):

e2save
OneStopPhoneShop
BuyMobilePhones
Mobile Shop
Simply 3G
Mobiles2YourDoor
Phones4u
Tesco
The Phone Spot
Dial a Phone
The Link
mobiles.co.uk
Carphone Warehouse

The retailers rely on the fact that some people will forget to claim their cashback, or will send in the wrong bill or do something else wrong - Make sure you aren't one of these people. Before you order, make sure you know what you have to do. When the phone has arrived, write the cashback dates down on your calendar (or use the mobile phones calendar or organsier to store the dates).

If the retailer sends you cashback "vouchers" with your phone, keep these really safe. If you lose them, they probably won't send you replacements.

Also, check around the newsgroups and forums on the internet. There's lots of information out there confirming that the cashback deals are genuine and detailing people's experiences.

What is 'automatic cashback' or 'upfront cashback'?
'Automatic Cashback' is money that the retailer will send you automatically, you don't have to claim anything. It's usually sent in installments, every few months.

'Upfront Cashback' is money that the retailer will send automatically, but the retailer will send it all at the beginning of the contract (sometimes it'll be sent with the phone itself, or sometimes it's sent when you've paid the first bill - That way the retailer knows you are going to honour the terms of the contract and that they will get their commission from the network).

How do these offers with free gifts work?
Different retailers do slightly different things - However, the general principle is that you have to sent off either your first, or first couple of phone bills to the retailer. They will then send you the gift within 28 days.

Recently, some retailers have started schemes that allow you to get the gift delivered with your phone. However, you usually have to give the retailer a deposit for this, which is then returned to you a few months later.

Be careful if you decide to change the tariff you are on, or cancel the phone contract before the minimum term is up as if you do this, the retailer will want to charge you for the free gift.

Phone up your network's customer services number, they should be able to tell you what you have to do.

To transfer your number to a different network, you'll need a 'PAC' (Porting Authorisation Code) - Customer Services should be able to give you this. When you sign up for your new mobile, either give the retailer your PAC or call your new network when the phone has arrived. Your number will then automatically move over a few days later.