Most likely they would be using shortcodes to send the notifications. This is kind of simplistic explanation but the reason I said that is that carriers are pretty restrictive in that SMS on regular phone numbers is to be used only for "person to person" communication and any kind of automated notifications have to be routed through a shortcode gateway which are more heavily controlled. This is basically to prevent SPAM/abuse as all the shortcodes have a clearcut opt out process to stop receiving messages (replying with STOP). So anything not send by a human is supposed to either use a shortcode gateway or e-mail gateway.
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