public function sort(Closure $callback)
    uasort($this->items, $callback);
    return $this;
Like so:
https://github.com/laravel/framework/blob/master/src/Illuminate/Support/Collection.php#L177
public function map(Closure $callback)
    $this->items = array_map($callback, $this->items);
    return $this;
https://github.com/laravel/framework/blob/master/src/Illuminate/Support/Collection.php#L188
public function filter(Closure $callback)
    $this->items = array_filter($this->items, $callback);
    return $this;
Otherwise you lose any manipulation you have done previously... Thoughts?
return $voicemails;
Both filter() and map() calls would be moot as they will be returning their own objects. You would have to do this to get it work properly:
$voicemails = $this->request('voicemail_count_url', array(
    // code
$voicemails = $voicemails->filter(function($voicemail) {
    // code
$voicemails = $voicemails->map(function($voicemail) {
    /// code...
return $voicemails;
          
Yes because those are fluent methods, you can just write it like that :
return $this->request('voicemail_count_url', array(
    // code
))->filter(function($voicemail) {
    // code
})->map(function($voicemail) {
    /// code...
          
Actually, I think both map() and filter() should be kept how they are (for a few reasons, sometimes you want new collections, also backwards compatibility)
However, map() has a similar function called each() which works on the current object and does not return a new one. I would suggest we add a similar companion function for filter().
https://github.com/laravel/framework/blob/master/src/Illuminate/Support/Collection.php#L158-L180
          
@Kindari Agreed. Although the descriptions for each() and map() should probably be updated to reflect what they actually do.
Maybe filtered() as the filter() companion?