I forgot my phone number in my old iCloud

As per the article Two-factor authentication for Apple ID - Apple Support

What to remember when you use two-factor authentication

Two-factor authentication significantly improves the security of your Apple ID. After you turn it on, signing into your account will require both your password and access to your trusted devices or trusted phone number. To keep your account as secure as possible and help ensure you never lose access, there are a few simple guidelines you should follow: 

• Keep your trusted phone number(s) up to date.

Visit your cellular service centre , provide personal identification proofs ( with you first and last name ) to them , you can obtain the trusted number .

Contact your cellular service provider , if luckily they have records of your number .

The Apple ID say example1@icloud.com exists on server and you know the password and its account credentials , however that is signed in the device iPhone .

If you don't remember the trusted number and yet it is verified in the account , visit your cellular service centre with personal identification proofs , the number can be obtained from them .

If the account is set with two factor authentication from iPhone .

Issue a sim on behalf of the same number and insert the sim Set up cellular service on iPhone - Apple Support

In a new iPhone that is purchased Turn on and set up iPhone - Apple Support

The Apple ID and password is active so is the 2FA active on the iCloud server , once a new trusted number is added the code will start appearing on the new trusted number , and the person can access the account .

See the article Get a verification code and sign in with two-factor authentication - Apple Support

Then access the account and remove an old inactive verified number from the account .

Go to your Apple ID account page and enter your Apple ID and password.

In the Security section, click Edit.

Click Add a Trusted Phone Number. 

To add a number, enter the phone number and verify it with a text. 

To remove a number, click cross sign next to the phone number you want to remove. 

Note : you don't need account recovery from the server as Apple ID and password are known to you How to use account recovery when you can’t reset your Apple ID password - Apple Support

Was a single trusted number was added in the account or second trusted number was also added , the 2FA account was created from where Mac or iPhone ?

Are the devices still used by you .