Учитывая следующий массив:
$array = [
[
"Row_Id" => 256,
"Start_Date" => "2020-05-16",
"account_code" => "",
"caller_number" => "452",
],
[
"Row_Id" => 257,
"Start_Date" => "2020-05-16",
"account_code" => "",
"caller_number" => "42",
],
[
"Row_Id" => 258,
"Start_Date" => "2020-05-16",
"account_code" => "",
"caller_number" => "428",
],
];
Собственный PHP
$array = array_map(function ($item) { return $item + ['callee_number' => 100]; }, $array);
Использование коллекций
$array = collect($array)->map(function ($item) { return $item + ['callee_number' => 100]; })->toArray();
Использование PHP 7,4 сокращений
$array = array_map(fn($item) => $item + ['callee_number' => 100], $array);
// Or
$array = collect($array)->map(fn($item) => $item + ['callee_number' => 100])->toArray();