We use email to SMS, a featured offered by some carriers. In order to do that, we need to know who the carrier is for each phone number entered. We send an email to a specific address depending on carrier, and the carrier then converts the email to an SMS message and sends it to the phone. This is typically a free service offered by carriers, at least in the USA. It may not be so in other parts of the world.
Because not all carriers support this feature and because email addresses are different by carrier, we require the carrier be selected. If you don't see the carrier you need, please contact us.