Im giving an old phone to wife questions.

How do I find if it is locked to a network had O2 sim previously,

Should I do a factory reset and how.

How can she keep her number she has a pay as you go sim

Thank you for any help

If its an Android phone - Settings - Backup and Reset - Factory Data Reset.

Switch on your mobile phone with a sim card from a network other than the network you are currently with. If you are able to use the phone, you can be sure that your phone is not locked. If your phone displays an error message saying "Wrong sim" or "Incorrect sim" or "Emergency calls only", etc., then your phone is probably locked.

If it is locked to one network its usually quite a simple process to get it unlocked. It will either be free with some software on one of the phone forums (I recommend Modaco) or you can buy an unlock code from Ebay, or take it to a indy phone shop that does repairs and cases and the like.

Her number is in her SIM, so it will transfer across to the new phone.

Here you go, there's a free option to unlock:

http://community.giffgaff.com/t5/Blog/Unlock-Samsung-Galaxy-S3-free/ba-p/7799058

Or a Play store app:

https://play.google.com/store/apps/details?id=com.spocky.galaxsimunlock&hl=en

Thanks Gibbo her old phone is steam driven and still has the big sim card not micro.

Ah, you'd need to visit your phone provider shop. They should be able to swap the number over to a micro SIM. They did with me on Orange.

Follow these steps.

1. Back up your data and settings to your Google account
2. Back up your photos and videos
3. Back up your Texts and call log
4. Encrypt your data (To encrypt your data, go to Settings > Security > Encrypt phone. )
5. Disable Reactivation Lock (in Settings > Security > and uncheck Reactivation lock.)
6. Perform a factory reset

Once you have all of your data backed up, it’s time to wipe it from your device. To ensure all of your data is gone, you’ll need to do more than perform a factory reset. In a recent study by Avast, the company found photos and other personal data on factory reset phones.

Source : Click me

If you feel confident with a craft knife large sims can be cut down to the required size or buy a cutter. Maybe someone on here has a cutter? When checking if the phone is unlocked you will need to put another sim in from a company not related to your original.

Again thanks to Gibbo and GeeMeister walked in Liscard Carphone Warehouse, explained as above the sim card was placed in a cutter 2 minutes later put in phone up and running total cost £4.99.
So WHY do we need all the malarkey about number transfers etc

Glad to have been of some help

Its a good idea to keep an eye on other service providers, their offers and prices. If you find a service that suits you from another provider you may wish to keep your old number. Check out Giffgaff.com, they piggyback onto O2 but are cheaper than O2 and have good coverage. Overall they are currently the cheapest sim only provider. To admin - Sorry if this is classed as advertising, I have nothing to do with Giffgaff, just offering a moneysaving idea.

your not from giff gaff so your recommending.

ive been using giff gaff for about 3 yrs now.
Love the unlimited data.

I watch for referral links

Thanks for the input Mark. I never bother with the referral link, im happy just referring. Ive been with them for many years and, like you, find the unlimited data excellent and the free giffgaff to giffgaff too as most of my kids and familly are on it - bargain.