Campaign phone numbers are like any other phone number. They can receive unwanted calls or text messages. Even though Campaigns do have a level of spam filtering, there are times when an unwanted message can still get through the system. 

Unsolicited messages are identifiable currently via the “+” sign next to the phone number in the activity pane for a given Campaign. 

These phone numbers may actually be spam, or they could be a message directed to a reusable number.