Explanation of IPhone Factory Reset

1. Nothing gets saved except the installed version of iOS.
2. No, your iPad contacts are separate.
3. No, your phone number is on the SIM card, so if you don't change the SIM card the number won't change.
4. You shouldn't get any surprises - except note that you will have to disable Activation Lock to factory reset the phone. If it wasn't your Apple ID that was used to enable it you will be surprised to find out that you can't use your phone ever again unless you know the Apple ID and password that were used to enable Find my iPhone.

If anything else occurs to you post back.

For what reason are you restoring your phone?

Nothing is saved when you restore to factory settings. Restoring to factory settings means restoring to out of the box condition. Restoring as new is what you will be doing. You should create a backup before you restore.

Your iPad does not share the iPhone number. It receives your text messages via text message forwarding from the phone. If you erase the phone, you will have to set that up again. The exception to this would be is if you are using iCloud messages. If you are, as long as you don’t delete any of the texts on either device, they will still be on the iPad.

None of this will affect your phone number. You will still have the same phone number assuming that you are going to keep the phone and set it up again or even get a new phone and set it up.

Restoring a device to factory settings or even erasing a device in the setting, will not have any effect on your other devices as long as you do not delete any content that syncs with iCloud.

Restoring to factory settings and/or erasing a device in the settings does not delete content from iCloud, only from the device itself.

You don't need to disable it if you restore to factory settings using iTunes, but you will need to enter the Apple ID password to set up the phone again. You do need to disable it if you use Settings/General/Reset-Erase all content and settings.