iCloud

When logging in to iCloud, a code will be sent to your trusted devices as a system message, unless you manually choose for the code to be sent to a trusted phone number either as an SMS or a voice call.

The use of the trusted phone number is optional unless you cannot access any other trusted Apple device to get the code from.

Link-> Two-factor authentication for Apple ID - Apple Support

If you cannot get the code in a trusted device, or through the trusted phone number you set up, then you will need to initiate an account recovery process to regain access to your account. Link-> How to use account recovery when you can’t reset your Apple ID password - Apple Support

In the future it is recommended to keep the trusted phone number up to date and accessible so you can get the codes there when you cannot access a trusted Apple device.