Me thinks your gut is leading you correctly. Generally speaking, there are short codes, and long codes. Sometimes alphanumerics are allowed, but the rules vary

Me thinks your gut is leading you correctly. Generally speaking, there are short codes, and long codes. Sometimes alphanumerics are allowed, but the rules vary regionally. Long codes resemble traditional phone numbers and tend to be treated as more disposable, low volume, and person-to-person. As such, they are typically easier to spoof with. Short codes tend to be more like car license plates: short random/vanity codes that require a more in-depth process to get access to. They are easier to verify ownership, suited for higher volume messaging, and tend to be backed by automation systems that respond to a set of automated commands like "HELP". These qualities make them less likely to be used for nefarious purposes. It's definitely a topic I'd love to understand more. Any corrections/additions are very much welcome!