Could you please tag a release?

The ApiSmsManager checks whether recipients' phone numbers follow the E.164 recommendation (lines 98 - 102):

foreach ($smsMessage->getRecipients() as $recipient) {
    Assertion::e164($recipient);
    $numberList->addChild('Number', $recipient);
    $hasAnyNumber = true;
}

It does this by pulling the whole Beberlei/Assert library and calling the method \Assert\Assertion::e164(), which by itself is nothing more than a small regex:

if (!\preg_match('/^\+?[1-9]\d{1,14}$/', $value))

My questions:

1. Is it worth it pulling down a whole library just for one regex? Maybe it would be better to drop the dependency and just check this manually, with your own method.
2. The E.164 is just a recommendation and it doesn't get followed. See Falsehood about phone numbers nr. 15:

ITU-T specifies that a phone number cannot be longer than fifteen digits, with one to three digits reserved for the country calling code, but valid numbers in Germany have been assigned that are longer than this.

Maybe this library could drop the Assertion dependency and just loosely sanity-check whether the given number looks like a phone number, something like

if ( ! preg_match('/^\+?\d+$/', $phoneNumber))

Checking if the number is a real phone number (let alone one which can receive SMS) is much more complicated and there is a whole huge library for it, so I would leave it out.

What do you think?

With help of the https://smsmanager.cz/api/xml documentation I was able to successfully send SMS with the new Apikey field.
Otherwise I'm getting Unauthorized exception. Was trying to use Email&Password and Email&Token combination.

        $requestHeader->addChild('Apikey', $this->password);

I'm testing this with a brand new SMSManager account, the old library LS/SMSManager is still working for LS.

In SendingFailed.php line 18:
                                                                                                  
  Sending failed for "+420xxxxxxxxx" because "The username or password you entered is incorrect"  

if (! (preg_match('/^\+420(?:(?:60[1-8]|7(?:0[2-5]|[2379]\d))\d{6})$/', $recipient)
    || (
        preg_match(
            '/^\+4219(?:0(?:[1-8]\d|9[1-9])|(?:1[0-24-9]|4[04589]|50)\d)\d{5}$/',
            $recipient
        )
        && $type !== RequestType::getRequestTypeLow()
    ))

Btw. not very readable, but why is the condition there at all?