How many minutes and texts do I need?
Many contracts now come with unlimited texts, so that's often not an issue. However, it's worth looking at your bills to see how many minutes you use and texts you send over a peak period - say Christmas - and making sure you choose a deal that suits. Don't forget about iMessages, though.
How long a contract should I take out?
Most smartphone contracts are now 24 months, so be prepared to keep the same handset for that time. Consider insurance as well to guard against losing your handset. 18-month and 12-month contracts are available with iPhones, though they're pricey up-front.
How much data do I need?
Providing you connect to Wi-Fi at the places you go to most (probably home and work), your data usage will be surprisingly low. You always feel that you need more data than you actually do. For most people, 250MB is more than enough. If you regularly visit websites and use web-intensive apps when out and about, at least 500MB is a safer amount.
Is it better to pay more upfront or more monthly?
Here's where you need to do some sums. If you pay £200 for the phone and then pay a monthly fee of £25, you will have paid £800 over 24 months. You could get the phone for free and pay £35 a month. But then you'd pay £840 over the term. Of course, the question is whether you can afford to pay out all that money up front.
What if I want tethering?
If you want to tether other devices to your iPhone's Personal Hotspot feature, you will need to pay close attention to the contract you take out. Some providers include tethering in their tariffs - such as Three's The One Plan - while others such as Vodafone will charge you extra when you first turn on tethering. O2 doesn't include data in its basic prices, but the Bolt Ons start from £3 per month and include tethering.
Can I keep my number?
Yes - see more on migrating to another network below.
Which deal is best for…
Getting a new 16GB iPhone 4S on contract
Best for talk
Three The One Plan: £99 upfront, £35 a month, 2,000 any-network minutes (plus 5,000 Three to Three minutes), 5,000 texts, all-you-can-eat data including tethering
Best for texts
Vodafone: £169 upfront, £31 a month, 300 minutes, unlimited texts, 500MB data
Best for data
Orange 41 Extra: £30 upfront, £41 a month, 600 any-network minutes, unlimited texts, 1GB data
Best for tethering
Three The One Plan: £99 upfront, £35 a month, 2,000 any network minutes, 5,000 texts, all-you-can-eat data including tethering
PAYG/SIM-Only plans for non-contract iPhones
Best for talk
Three The One Plan SIM-only: £25 a month, 2,000 any-network minutes (plus 5,000 Three to Three minutes), 5,000 texts, all-you-can-eat data including tethering
Best for texts
Vodafone Freebie SIM: Top up with £30 and get £30 credit plus 3,000 UK texts and 500MB of data
Best for data
Giffgaff £10 Goodybag: £10 per month, 250 any-network minutes, unlimited texts, unlimited data
Best for tethering
O2 Simplicity: 100 minutes £10.50 a month plus £10 for 1GB including tethering, 100 minutes (plus 100 O2 to O2 minutes), unlimited texts
What else do I need to know?
It's possible to buy an iPhone outright without a contract and then get a cheap monthly or PAYG deal for it. Apple sells the iPhone 4S at £499 for 16GB, £599 for 32GB and £699 for 64GB. These are unlocked handsets.
You can also buy an iPhone from other sources such as eBay, of course, but make sure it's unlocked or that you'll be using it on the same network it's locked to - each network has its own way to get your handset unlocked after a contract ends, but they all have help pages.
Buying a SIM-free handset can be a surprisingly reasonable way of doing things. If you pay for a phone SIM-free from Apple (£499 for the 16GB iPhone 4S) and then £10 a month PAYG, over two years that costs a total of £739. If you get a free handset but it costs £36 a month, though, you'll pay £864 over the two years.
For SIM-only deals, you can try the major providers, but there are some great deals available from relative unknowns Three, Tesco Mobile as well as a network you might never have heard of; Giffgaff. It actually uses O2's network, but offers some great data and talk bundles, including unlimited data on some tariffs - something that the major networks have turned their back on.
If you're getting a SIM card for the iPhone 4 or 4S, make sure you tell your network, so they send you the smaller microSIM card.
Selling your old handset
Whatever phone you had before your new iPhone, it's worth checking its trade-in price - you never know how much you might get, and most sites will even provide a padded envelope to send your phone to them in. You won't need to send in your box, charger, headset or manuals - just your handset itself.
We have a mobile phone recycling comparison engine, while there are other sites such as sellmymobile.com and fonebank.com. The great thing about iPhones is that they keep their value extremely well - an iPhone 3G 8GB could be traded in for £72 at the time of writing; an 8GB 3GS fetched £100.
If you do trade in your old phone, make sure you erase it first. With an iPhone, go to Settings > General > Reset.
Migrating to other networks
Whether you take up a new contract or buy a phone to use on pay monthly or PAYG, you can always keep your number. Of course, you must make sure your current contract has run its course before you can cancel it and take out a new one.
To cancel it after this and keep your number, call your network and ask for your PAC - your Porting Authorisation Code. This essentially means that your new network can access your number. Most networks will text or email this to you, though some still send it by letter to delay your decision!
When you take out your new contract or order your new SIM, enter your PAC on the website or tell the assistant you want to transfer your number. When your number is transferred to your new provider, your old contract will cease to exist and you'll receive a final bill.
Keeping track of your data usage
It's important to keep an eye on how much mobile data you're using - especially if you have a contract with limited data included. You can check how much data you're using within iOS - go to Settings > General > Usage > Mobile Usage and there you can see how much data you've sent and received, as well as how much data you've used over tethering. You can reset these stats whenever you want.
Many mobile networks including O2 and Vodafone also have their own apps that help you see how much data you've used since your last bill.
Your iPhone comes with a one year warranty. If you get a problem with it during this time, the best thing to do is take it to the Genius Bar in your nearest Apple Store. Make sure you back it up in iTunes or on iCloud first - if the assistant performs an exchange, he or she will wipe your handset in front of your face!
If you're out of warranty, it's worth taking it in anyway. We've heard of manufacturing defects resulting in a replaced handset, even if it's old. Your network may not be so accommodating, but it's worth looking at the Sale of Goods Act, which enables you to complain if you're still in contract but the phone is not 'fit for purpose'.
For complete peace of mind, it's worth checking out the AppleCare Protection Plan for iPhone. It costs £61, extends the warranty to two years and gives you full phone support.
If you're buying a device as expensive as an iPhone 4S, you'll want to insure it. Many bank accounts and insurance policies offer included or cheap mobile phone insurance these days, so that may be one way of doing it, but you can get a decent separate deal for only a few pounds per month - as ever, use an online comparison site to get the best deal.
Be very wary of buying it at the point of purchase - many deals you buy at the same time as your contract are expensive, and you end up paying more than you need to. Be aware that some insurance companies charge a higher excess fee for iPhones - a consequence of the their huge popularity.
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Dan (Twitter, Google+) is TechRadar's Former Deputy Editor and is now in charge at our sister site T3.com. Covering all things computing, internet and mobile he's a seasoned regular at major tech shows such as CES, IFA and Mobile World Congress. Dan has also been a tech expert for many outlets including BBC Radio 4, 5Live and the World Service, The Sun and ITV News.