Who I have the same questioned this topic

My daughter is on a Just Kids line, and we've seen her receive at least 2 spam text messages ("...you've won a $1000 Wal-Mart gift card...") from numbers we definitely don't have on the trusted contacts list.

One was from a group text message, but the other was a single text message direct to her line.

We're on an Android Galaxy S7 Edge, so it is not an iMessage style issue. Is there any reason why messages can get through from numbers that are not in the trusted contacts list?