That's the short version. The longer one takes each piece in turn, with the source for each so you can read it yourself.
Why you can't just send
This article covers what the carriers ask, what the FCC's rule says, and what CTIA's guidance says. The carriers, the phone companies that deliver the texts, ask the business to register. The FCC has a rule about consent.
Both are explained below with a link to the source. This is general information, not legal advice, and a lawyer should read anything that is specific to your state or your messages.
Register the business
Microsoft's documentation on 10DLC guidelines says businesses must complete brand registration and campaign registration. 10DLC is the name for this kind of sending, short for 10-digit long code. Those are the two steps to ask your provider about.
The registry behind it is The Campaign Registry. Its own site says senders are verified before they are allowed to send, and that direct registration is not available to businesses. You go through a messaging provider.
Twilio, a messaging provider, says anyone sending texts over a 10DLC number from an application to the US must register. So the first question for any texting tool is whether your business is registered through it.
Get consent, and let people leave
Registration tells the carriers who you are. Consent is a separate matter, and it comes from the people you text.
The FCC's rule sets out when prior express written consent is needed for marketing calls made with an automatic dialer, and it defines that as a signed written agreement. Whether and how that applies to your texts is a question for your lawyer. If your texts are appointment reminders or updates, ask a lawyer or your provider what applies.
Then comes the way out. The same rule says a person can revoke consent, and it names words that count: stop, quit, end, revoke, opt out, cancel, or unsubscribe.
CTIA publishes its own guidance in its Messaging Principles and Best Practices. It is guidance, not law. It says to use standard STOP wording in your opt-out instructions, and to act on plain requests like "unsubscribe" or "please opt me out."
Keep the records
CTIA's guidance says senders should keep opt-in and opt-out records, so that a later message isn't attempted to someone who opted out. CTIA's guidance doesn't set how long to keep them. Ask your lawyer about that.
Questions for your texting tool
Put the pieces together and you have a short list of questions for any tool that sends texts for you. Is the business registered through it? When someone texts stop, does it stop, and does it recognize the plain-language ways people ask? Does it keep opt-in and opt-out records?
If the answer to any of them is no, fix that before the first text goes out.
What we do
Registered texting is live in RVDesk and the RVDesk Scheduler, the scheduling part of RVDesk on its own: customer intake, messages, booking links and the customer's message thread. Reminders and updates go out by text and email, with the opt-out rules handled. It is not live in every tool we have shipped.
The carrier registration, the opt-out handling and the message log are built once, and available to every build we do. Our trust page says which services handle texts and email.
When you don't need us
If a texting service you already use registers your business, handles stop requests, and keeps opt-in and opt-out records, use it. Consent is a question to put to your lawyer or your provider. You don't need us for that, and we'll tell you so.
If you text a few people by hand from your own phone, you aren't running automation. Ask your provider how the rules treat very small senders.
Where we come in is when texting is part of how the business runs: reminders that go out on their own, anything a customer is meant to receive without you pressing send. Software that just runs is what that looks like.