Apple's latest iPhone, the 3GS, comes out on Friday.

If you're thinking of buying one or you've already decided the 3GS is the phone for you, here's everything you need to know: where to get one, what to pay and what to do with your old one.

So what's so great about the 3GS?

It's the latest version of Apple's much-hyped iPhone, with a faster processor, more storage, a better camera and the ability to bring your ancestors back from the dead.

Can you be a bit more specific?

Certainly. Check out our in-depth look at all the new iPhone 3GS features.

Is there more than one model?

Yes. The iPhone 3GS comes in two versions: a 16GB model and a 32GB model.

Can I get it in pink?

No, but you can choose between white and black versions. Unlike the 3G, both versions of the 3GS are available in both colours.

Is there more than one network?

No. In the UK, the iPhone 3GS is exclusive to O2.

I'm not with O2 for my current phone. Can I keep my old number?

Yes, as long as you're not still in the minimum contract period for your existing phone. Contact your network and ask them for a PAC code. You'll need this to transfer your number to O2. The process usually takes a couple of days, so you'll have a temporary number in the meantime.

Can I upgrade my iPhone 3G?

Yes, but it won't be cheap. If you signed up for an iPhone 3G last summer, you'll need to pay to end your contract early and sign up for a whole new contract, or pay full whack for an iPhone 3GS on Pay as you Go.

Pay to end my contract? How much?

Officially, the buyout will be however many months you have left on your contract multiplied by the monthly tariff - so if you've got six months left on a £35/month tariff that means £210 to end the contract.

You'll then pay the same prices as any other iPhone 3GS customer. O2 promises to review each case individually, but if you've got months left on your contract don't expect O2 to let you off the hook for a tenner.

Is there anything I can do to end my contract early?

If you're not willing to pay, O2's priority list offers early upgrades for big spenders. If you spend more than £35 per month every month you can upgrade one month early; £50+ per month gives you an upgrade three months early; and £80+ per month entitles you to an upgrade six months early.

Are there any special deals for existing iPhone customers?

Not yet. TechRadar reported that Carphone Warehouse will offer up to £160 cashback on old iPhones, but if it's doing it it hasn't started yet. Even if it does, you'll still need to pay to end your existing contract.

You could always buy a Pay as You Go and sell your 3G on eBay: at the time of writing the going rate for a 3G appears to be around £300. Of course, if everybody's flogging iPhone 3Gs from Friday then the going rate is going to drop dramatically.

Can I get one on Pay As You Go?

Yes. The 16GB model will be £440.40 and the 32GB model £538.30. The price includes 12 months of free Wi-Fi and Web access.

Can I get one if my credit rating isn't great?

On Pay as you Go, yes. On contract, it depends. You'll need to bring proof of address and have a valid debit or credit card, and while O2 will credit check you, phone networks aren't as strict with their checks as other retailers.

What does it cost with a contract?

That depends on the contract. O2 has provided a table to explain the options:

iPhone tariffs

Where did the 8GB come from? I thought the 3GS was 16GB and 32GB?

You're right. The 8GB model is the 3G, not the 3GS. In other words, it's the old phone.

So what's the best tariff to go for?

Don't fall into the trap of going for a really expensive tariff just because the phone's free: if you get a free 32GB phone on a 24-month contract, you'll pay £1,761 over two years. On the cheapest 18-month contract you'll pay £274.23 for the phone, but the total cost including that up-front fee is only £803.

Look at your existing phone usage and pick a tariff that offers the right amount of minutes and texts but not too many.

Does that include using my iPhone as a modem?

No. That's an additional £14.68 per month for 3GB of data, or £29.36 per month for 10GB.

I thought the monthly contracts included unlimited data?

They do, but apparently using your phone as a modem is the wrong sort of data. O2 isn't picking on iPhone owners here: the same rules apply to its other smartphone contracts.

Can I use a Pay as you Go iPhone as a modem?

No.

Where can I get one?

You can buy online from O2, Apple or the Carphone Warehouse, or you can visit an O2, Apple or Carphone Warehouse shop. At the time of writing Apple only sells Pay as you Go models online.

Can I pre-order before Friday?

Yes, but only on the Carphone Warehouse website.

I don't want to wait for delivery. Where's my nearest shop?

You can buy the 3GS from three different shops: O2, Carphone Warehouse and Apple. Each firm very helpfully offers a store finder. Click on the appropriate link:

Find my nearest O2 shop

Find my nearest Carphone Warehouse Shop

Find my nearest Apple Store

Will I be able to pick up a 3GS in a shop before Friday?

No.

Can I activate the iPhone if I don't have a PC?

No. The iPhone needs iTunes like a fish needs water. You need iTunes 8.2 to activate your iPhone or to sync data with your computer. Without iTunes, you've just bought an expensive ornament.

Should I rush out and buy one first thing on Friday?

We wouldn't recommend it. Past experience suggests that retailers' systems will struggle with demand, and that some shops - particularly smaller ones - might not have enough stock on day one.

We won't name the shop, but when we bought our iPhone 3G on launch day last year the majority of iPhones had already been reserved for employees' friends and family.

Should I rush out and buy one full stop?

Good question. It's obvious that Apple plans to bring out a new iPhone at least once a year, but the contracts O2's offering are for eighteen or twenty-four months.

Upgrading to the next iPhone won't be cheap, so if you're already on a 3G you should check out the 3.0 software update first. It might be smarter to stick with the 3G just now, skip the 3GS and get the next iPhone instead. Apparently it will include a 72-inch OLED screen, terabytes of memory and a free horse.

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Liked this? Then check out All the new iPhone 3GS features

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