It might vary. When you get a spam text, you can use something like Twilio's number lookup to find the carrier.

If you ever get a suspicious/spam text, looking up the carrier is a good first step. Most of the garbage I get comes from VoIP numbers because they can easily spin up disposable numbers from places like Telnyx or Bandwidth.com. That's not to say someone can't be using an actual mobile phone, but usually it's coming from some VoIP system.