您可以选择创建自定义验证规则来验证电话号码的唯一性。

php artisan make:rule UniquePhoneNumber

app/Rules/UniquePhoneNumber.php

<?php
 
namespace App\Rules;
 
use Closure;
use App\Models\User;
use Illuminate\Contracts\Validation\ValidationRule;
 
class UniquePhoneNumber implements ValidationRule
{
    /**
     * Run the validation rule.
     */
    public function validate(string $attribute, mixed $value, Closure $fail): void
    {
        return User::where('phone_no', request('calling_code') . '-' . $value)
                    ->doesntExist();
    }
}

然后使用新的自定义验证规则：

use App\Rules\UniquePhoneNumber;

public function customRegistration(Request $request)
{
    $request->validate([
        'name' => 'required',
        'email' => 'required|email|unique:users',
        'country' => 'required',
        'phone_no' => ['required', new UniquePhoneNumber],
        'password' => 'required',
    ]);

    $data = $request->all();
    $check = $this->create($data);

    return redirect('login')->with('message', 'Signing up successful !!!');
}

编辑：

（感谢@brombeer，我注意到你通过连接插入值，同时将初始值与数据库中已经连接的值进行比较，这意味着你没有在数据库中强制唯一性）
所以你可以用漂亮的可接受的答案来做，或者你可以用验证来做，

'phone_number' => [
        'required',
        function ($attribute, $value, $fail) use ($request) {
            $callingCode = $request->input('calling_code');
            $phoneNumber = $callingCode. '-' . $value;
            $exists = User::where('phone_number', $phoneNumber)->exists();

            if ($exists) {
                $fail('The ' . $attribute . ' has already been taken.');
            }
        },
    ],