Yes. The to parameter can be a SPACE Separated List of numbers in International format.. For example: to=642715414141 6178542544 12059955852
Ensure you URLencode the parameters.
Mobile Number Portability (MNP) allows mobiles to change networks while retaining their phone number. Bulletin receive a file each day which contains details of numbers that have ported. This file is provided by a third party working in association with the carriers and it generally takes 2-3 days for a ported number to appear in this file after a number has changed networks. During the short time between the handset changing networks and us being told of the change, the handset will not receive messages.
Any long term issues with ported numbers should be raised with Support.