I forgot the two-factor authentication phone number on the other iCloud account

The other account if Apple ID is not signed in the device and Apple ID and password is known .

Use a web browser and open iforgot.apple.com

Enter only Apple ID address in the field , enter the characters shown and click on continue .

Another pop up window appears the last two digits number that was added in the 2FA account will be shown .

You can visit your cellular services , give personal identification proofs the number can be obtained from them .

If old Apple ID is signed in the device , you forgot the number see the article Manage two-factor authentication for your Apple ID from iPhone - Apple Support

View or remove trusted devices

1. Go to Settings  > [your name].
2. A list of the devices associated with your Apple ID appears near the bottom of the screen.
3. To see if a listed device is trusted, tap it, then look for “This device is trusted and can receive Apple ID verification codes.”