private function addRecipient($recipient)
    {
        if (is_null($recipient)) {
            throw new InvalidArgumentException('Recipient cannot be NULL');
        }

        if (!is_string($recipient)) {
            throw new InvalidArgumentException(
                sprintf(
                    'Recipient must be presented as string, but "%s" given',
                    is_object($recipient) ? get_class($recipient) : gettype($recipient)
                )
            );
        }

        if (\preg_match('/^\+\d+$/Ds', $recipient) != 1) {
            throw new InvalidArgumentException(
                sprintf(
                    'Recipient must match pattern \+\d+, but "%s" does not match',
                    $recipient
                )
            );
        }

        array_push($this->recipients, $recipient);
    }
private function addRecipient($recipient)
    {
        if (is_null($recipient)) {
            throw new InvalidArgumentException('Recipient cannot be NULL');
        }

        if (!is_string($recipient)) {
            throw new InvalidArgumentException(
                sprintf(
                    'Recipient must be presented as string, but "%s" given',
                    is_object($recipient) ? get_class($recipient) : gettype($recipient)
                )
            );
        }

        if (\preg_match('/^\+\d+$/Ds', $recipient) != 1) {
            throw new InvalidArgumentException(
                sprintf(
                    'Recipient must match pattern \+\d+, but "%s" does not match',
                    $recipient
                )
            );
        }

        array_push($this->recipients, $recipient);
    }
@param string $recipient A phone number in the one of formats: "+71234567890" or "71234567890".

@throws \InvalidArgumentException If $recipient is not a string or format is invalid.