how do i unlock old phone im not currently using ?

Passcode forgotten:

See these instructions, keeping in mind that if you don't have a backup all data will be lost on that device (except those that you eventually synced to iCloud):

If your device is running iOS 15.2/iPadOS 15.2 or later, AND if you have created a backup to iCloud, you don't need a computer:--> How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

Shouldn't be this case (iOS/iPadOS earlier than 15.2), you need a computer. If you don't have a computer, borrow one from your friends/relatives/neighbors or ask for help at an Apple Store or Authorized Service Provider, then read these suggestions.

Given that to remove the existing passcode you must connect your device via USB to a computer (a Windows PC running the latest iTunes version, or a Mac, which can run iTunes or Finder, depending on the macOS installed on it), pay attention to the specific gestures that you need to perform while connecting your device to the computer.

These are the instructions to re-gain access to your iPhone/iPad:

If it is an iPhone:--> If you forgot your iPhone passcode - Apple Support

If it is an iPad:--> If you forgot your iPad passcode - Apple Support

Also, take a look at this support video about the same matter (for both iPhone and iPad):

https://www.youtube.com/watch?v=7s8-zLG9ySQ

Find My [device]:

Find my app (for both iPhones) is working if you use the same Apple ID on the old and new device.

Regards

Giulio

Locked?

Unlock 1 = Apple ID locked (Generic):-> If your Apple ID is locked or disabled - Apple Support

Unlock 2 = Apple ID locked (in AppStore&iTunes):->  If you see a message that says 'Your account has been disabled in the App Store and iTunes'

Unlock 3 = Passcode forgotten:-> If you forgot the passcode on your iPhone, or your iPhone is disabled

Unlock 4 = SIM not supported:-> How to unlock your iPhone for use with a different carrier - Apple Support

Unlock 5 = SIM PIN:->  Contact your carrier asking them for your SIM PUK code

Unlock 6 = Activation Lock:-->  How to remove Activation Lock

Unlock 7 = Frozen device:--> If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

If you mean something different, please better specify your needs.

Regards

Giulio