foxxx1234

$calls = [
    ['prefix' => '+57'],
    ['prefix' => '+268'],
    ['prefix' => '+972'],
    ['prefix' => '+972'],
    ['prefix' => '+33'],
    ['prefix' => '+33'],
    ['prefix' => '+972'],
    ['prefix' => '+509'],
    ['prefix' => ''],
    ['prefix' => '+33'],
    ['prefix' => '+268'],
    ['prefix' => '+57'],
];

function calcPrefixStat($calls) {
    $stat = [];
    foreach($calls as $call) {
        if (!array_key_exists($call['prefix'], $stat)) {
            $stat[$call['prefix']] = 0;
        }
        $stat[$call['prefix']]++;
    }
    
    return $stat;
}

var_dump(calcPrefixStat($calls));
/*
array(6) {
  ["+57"]=>
  int(2)
  ["+268"]=>
  int(2)
  ["+972"]=>
  int(3)
  ["+33"]=>
  int(3)
  ["+509"]=>
  int(1)
  [""]=>
  int(1)
}
*/