I have 4 models held together by 4 pivot tables:

User, Company, Phone, Address

The User and Company are both tied together to Phone with the pivot tables user_phone and company_phone.

User has a method: addDefaultPhone( Request $request ). It grabs $request->input() and creates a phone number and attaches it to the User.

The same exact method can be used in the Company class. How can I include addDefaultAddress to both classes (without copying and pasting of course)?

You may use inheritance or trait. Choose whatever you want.

Inheritance

Create a base class:

namespace App;
use Illuminate\Database\Eloquent\Model;

class ThingWithAPhone extends Model {
    public function addDefaultPhone() {
        /* Whatever you want... */
    }
}

Then both User and Company could extend this class:

// In the user file
class User extends ThingWithAPhone { /* ... */ }

// In the company file
class Company extends ThingWithAPhone { /* ... */ }

Trait

Create a trait:

namespace App;

class Phonable {
    public function addDefaultPhone() {
        /* Whatever you want... */
    }
}

Use this trait:

// In the user file
class User extends Model { 
    use Phonable;
    /* ... */ 
}

// In the company file
class Company extends Model {         
    use Phonable;
    /* ... */ 
} }

With trait you can create class which have X traits, and one other class with Y traits and some of these traits may be common to both classes.

The Best i can think of is to create a trait and share among the classes.