<?php
$arr = [
1 => '79066692770',
2 => '79516943359, 89676667788',
3 => '+79107835988',
4 => '+79510820663 , 89991231221, 8123980912'
];
$res = array_map(
fn($el) => array_map(
fn($el)=>['VALUE'=> $el, "VALUE_TYPE" => "WORK"],
explode(',', $el)
),
$arr
);
print_r($res);
//получить
/*
$ar2 = (([1] => (("VALUE": "79066692770", "VALUE_TYPE": "WORK")),
[2] => (("VALUE": "79516943359", "VALUE_TYPE": "WORK",
"VALUE": "89676667788", "VALUE_TYPE": "WORK")),
[3] => (("VALUE": "79107835988", "VALUE_TYPE": "WORK")),
[4] => (("VALUE": "79510820663", "VALUE_TYPE": "WORK",
"VALUE": "89991231221", "VALUE_TYPE": "WORK",
"VALUE": "8123980912", "VALUE_TYPE": "WORK"))))
*/