How to create a pivot table with related field other than id on Laravel 5.2

I want to create (on Laravel 5.2) a pivot between two entities, let's assume that we have the entities User and Phone. In our world, a user can have several phones, and te phones can have several owners.

For a reason, I must target the user's phone by it's serial number and the build year (let's assume that serials can't be identical on a same year).

So I have my tables users, phones andusers_phones.

===== Table users =====
id
username
...

===== Table phones ====
id
serial_numer
build_year

===== Table users_phones ====
user_id
phone_serial
phone_build_year

I have my relation on my model Phone :

public function users()
{
    return $this->belongsToMany(
            User::class,
            'users_phones',
            'user_id',
            'phone_serial'
        )->withPivot('phone_build_year');
}

When I wan't to add some entries like that :

$phone->users()->attach([$user->id => [
    'phone_build_year'  => $phone->build_year
]]);

The SQL request generated is the following one :

insert into `users_phones` (`user_id`, `phone_serial`, `phone_build_year`) values (1, 5, 2016)

My problem is that the 5 in the sql request is the phone id and not its serial. I didn't find how to precise to Laravel to use the field we chose (here the serial) rather than the id.

Is it possible to do that, or am I condemned to create a model to manipulate my special pivot table ? I admit that I would appreciate if I could avoid that way.

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