Locked iCloud Account

Read the article:

Two-factor authentication for Apple ID - https://support.apple.com/HT204915

thoroughly. Various sections contain information that is key to you being able to set up and use a different telephone to get a verification code you can use.

"If your iPhone is your only trusted device and it is missing or damaged, you will be unable to receive verification codes required to access your account."

Read the sections about:

- Trusted phone numbers

- Manage your account > Keep your trusted numbers up to date and add a phone number.

- What if I can't access a trusted device or didn't receive a verification code?

And if necessary:

- If I can't sign in, how do I regain access to my account?

Sign into your account at https://appleid.apple.com/ and add your new number, then sign out of iCloud and back in again on your devices.

Get a verification code and sign in with two-factor authentication - https://support.apple.com/HT204974

Get a verification code and sign in with two-factor authentication - https://support.apple.com/HT204974

Recover your Apple ID when you can’t reset your password - "If you use two-factor authentication and can’t sign in or reset your password, you can regain access after an account recovery waiting period." - https://support.apple.com/HT204921

Not necessary the account recovery is to be done every time , if one remember the old trusted number ( say 1234.... ) that is inactive , the person can visit the cellular service give his identification proof and issue a sim on behalf of the same number and insert the sim Set up cellular service in new iPhone that is purchased

Turn on and set up iPhone

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  https://support.apple.com/en-us/HT204974

The Apple ID say example1@icloud.com exists on server and you know the password and its account credentials , however that is not signed in the device iPhone / Mac / iPad .

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

The account was not set up with secondary authentication in which three answers for respective questions , rescue mail address and its password , date of birth is added .