Most scenarios are covered by a hasOne, hasMany or ManyToMany relation. How to use them is explained in detail here. I give a brief overview how they are connected to the database.
User hasOne Phone
You should have two tables that look somehow like this:
From the user class you can access his phone as
And you can go back from your phone class to the related user:
User hasMany Phones
Since a user may have multiple phones, one needs to equip each phone with an unique id.
From the user class we only need to change hasOne to hasMany
In the phone class we do not need to make any changes.
User hasMany Phones & Phone hasMany Users
In this scenario we need a pivot table
Now both directions (back or forth) are pretty much the same and look like this in the user class:
and in the phone class: