How to send JSON request with guzzle6 and Laravel - json

Question

From an API I need to send the request in this format:
Sample Request Body
{
"first_name": "adam.west",
"last_name": "adam.west",
"email_address": "adam.west#example.com",
"phone": {
"number": "11111111111",
"country_code": "UK",
"phone_type": "Mobile",
"extension": "124"
},
"comment": "comment",
}
so with Laravel and Guzzle6, I write this:
$res1 = $client1->post('https://example.com/someurl', [
'headers' => [
'Content-Type' => 'application/json',
'Authorization' => 'Bearer '.$token,
],
'json' => [
'first_name' => $FirstName,
'last_name' => $Surname,
'email_address' => $Email,
'phone' => [
$phone['number'] = $Mobile,
$phone['country_code'] = $MobileCountryCode,
$phone['phone_type'] = 'Mobile',
$phone['extension'] = $request->dialCode,
],
'comment' => $comment,
]
]);
Everything is fine except phone element - What is the proper way to send phone element - because from API response I got error message "Phone could not be empty"