Do you pay your mobile phone bill month after month without checking for a better deal? Many people do, and it's partly because they think it's complicated and time-consuming to switch to a new provider. In fact, it's surprisingly quick and easy. Plus, as long as you're not still locked into a contract, it's free.

Find a new tariff

First, choose a new contract or pay-as-you-go (PAYG) deal. This could include both a phone and SIM, or just the SIM if you want to keep your existing handset. It doesn't matter if you're moving from a contract to PAYG, or even the other way around, as the process for taking your number is exactly the same.

Bear in mind that the new SIM will have a new number, but your old number will replace it when the transfer takes place. Hold fire on actually ordering the SIM or phone as you might be able to get your current supplier to match the deal.

If you're coming to the end of a contract, check the small print to find out how much notice you need to give to leave the network. Even after the contract finishes, you may still have to give one month's notice that you're cancelling.

Get the PAC

The next step is to phone your current mobile provider to request a port authorisation code (PAC). Mobile operators are legally obliged to provide the code within two hours, but you'll typically be given the ‘customer retention' treatment with offers to persuade you to stay with the network.

You may well end up with a better offer than the new package you've chosen, in which case you won't need to transfer your number. It doesn't matter if there's no offer or the new deal can't be beaten as you can simply ask to be given your PAC.

PAC text

Have a pen and paper handy in case the number is given to you over the phone, but most networks will send it to you in a text message. Some may post it to you, so bear this in mind if you're in a hurry. The code should be in the format ABC123456 and is valid for 30 days.

Once you have a PAC, you can give it to a friend or family member so they can have your old number. It isn't tied to a particular person or address.

Order the new SIM

Now's the time to order your new phone or SIM. Depending on where you buy it from, you may be able to enter the PAC (or say that you will be keeping an existing number) as part of the sign-up process.

Otherwise, while you're waiting for your new phone or SIM to be delivered, call the new provider's customer services, tell them you want to keep your current number and give them the PAC. They will do the rest, and should tell you when the number will be switched over.

Back up your contacts

You can continue to use your old SIM until the day of the transfer, but make sure you back up any contacts and text messages that are stored on the SIM otherwise you'll lose them. If you can't export the contacts from your phone to your computer using the phone manufacturer's software you might have to write them down by hand.

It's worth paying a visit to the old or new mobile operator's store as they can usually copy the contacts and SMS messages for you. Once the number transfer happens, the old SIM is deactivated and won't work in any phone – it's good only for the bin. Similarly, make sure you use up any remaining credit or you'll lose that too.

On the day

If you can, choose a day for the transfer when you don't need to rely on your phone. This is because there could be a gap in service between the time the old SIM is switched off and the number is activated on the new one.

Of course, you can start using your new SIM when it arrives – and tell people it's a temporary number – but this may stop working several hours before your old number is transferred to it.

It should never take more than 24 hours for the transfer to complete: it can be over in as little as a few minutes. Typically, you'll be without service for an hour or so.

Useful numbers

Here are the numbers to call from your mobile to get a PAC from each of the five main networks:

Three: 333
O2 – Pay Monthly: 202
O2 – Pay And Go: 4445
Orange – Pay Monthly: 150
Orange – Pay As You Go: 450
T-Mobile: 150
Vodafone: 191