Q: Can I change my telephone on icould keychain?

Well, what do the instructions say if you cannot access the number?

From Frequently asked questions about iCloud Keychain - Apple Support :

If you can't access any of your other devices that are using iCloud Keychain, you can still set up iCloud Keychain on another device if you have these items:

• Your iCloud Security Code
• The device that is using the SMS-capable phone number you provided when you first set up iCloud Keychain. A verification code is sent via SMS to this phone number. If you can't access this number, contact Apple Support, who can verify your identity, so that you can complete setup on your new device.

You might also want to read:

Get help using iCloud Keychain - Apple Support