MSID stands for Mobile Station ID. According to Wikipedia, MSID is a number that is associated with the home service provider and the wireless phone number. This is reprogrammed when the user changes home service providers. To find out what you're MSID is, contact your service provider. Your MSID could be the same as your phone number. It will be different if you ported your number from another service provider to your current one. I hope this helps. source