Three directions#
Messaging people sort traffic by who sent it and who receives it:
- A2P — application to person. Software sends to a human. Examples: a bank's one-time passcode, a delivery update, an appointment reminder, a promotion.
- P2P — person to person. One human texting another from their own phone.
- P2A — person to application. A human writes to software: a customer replying "YES" to confirm, texting a keyword, or asking a question that lands in a support inbox.
These look like jargon. They are the reason your messages cost what they cost and arrive when they arrive.
| Direction | Who starts it | What it costs you |
|---|---|---|
| Application to person | Software, to a human | Registration, filtering, a business rate |
| Person to person | A human, to a human | A consumer plan, and none of the above |
| Person to application | A human, to software | The fewest constraints of the three |
Why the distinction exists#
Mobile networks were built for people texting people, and priced that way — bundles, flat rates, unlimited plans. Then businesses discovered they could reach anyone for the price of a personal text. The economics stopped working, and the abuse arrived: at scale, a channel with no gatekeeper becomes a channel nobody reads.
The response was to separate the traffic. A2P is identified, registered, filtered and priced differently from P2P. That one decision explains almost every rule you will meet.
How business traffic came to be separated
- 01Built for people texting people
Priced that way: bundles, flat rates, unlimited plans.
- 02Businesses discover the price
Anyone can be reached for the price of a personal text.
- 03Abuse at scale
A channel with no gatekeeper becomes a channel nobody reads.
- 04The traffic is separated
A2P is identified, registered, filtered and priced differently from P2P.
What follows from being A2P#
You must be identified. Most markets now require businesses to register who they are and what they send. The details differ:
| Market | What registration looks like |
|---|---|
| United States | Register a brand and a campaign for ordinary numbers ("10DLC"); unregistered traffic has been blocked by carriers since February 2025 |
| India | Register the business, its sender headers and message templates on operators' DLT (distributed ledger) platforms |
| Singapore | Register alphanumeric sender names with the national SMS Sender ID Registry; unregistered names are blocked |
| United Arab Emirates | Register sender IDs; promotional messages carry a mandatory "AD-" prefix |
Other countries have their own versions, and they change. See sender registration around the world.
Your content is inspected. Operators filter business messages for spam and fraud. The rules are mostly unpublished — deliberately, because a published filter is easy to evade.
Your price reflects it. A2P rates are business rates, not consumer rates. The gap is how operators recover what unlimited plans gave away.
Sender types and what they signal#
The "from" field matters for cost, speed and whether people can reply.
| Sender type | What it is | Good for | Watch out for |
|---|---|---|---|
| Long code | An ordinary phone number | Two-way conversations, low to moderate volume | Throughput limits; registration in some countries |
| Toll-free number | A free-to-call number (in North America, for example) | Two-way at moderate volume | Its own verification process |
| Short code | A 5–6 digit number leased for business use | High volume, fast delivery | Expensive and slow to obtain; country-specific |
| Alphanumeric sender ID | Your brand name instead of a number | Recognition | One-way: recipients cannot reply; not supported in some countries, such as the US |
The alphanumeric sender is the classic trap. A brand picks it for recognition, then discovers it has designed out the cheapest and least restricted direction of all: replies.
The volume each number type is good for
- 1Long code
Two-way conversations, low to moderate volume
- 2Toll-free number
Two-way at moderate volume, with its own verification
- 3Short code
High volume and fast delivery — expensive and slow to obtain
Why P2A is the interesting one#
Inbound is the direction with the fewest restrictions, because the customer chose to start it. Almost every constraint on this site — WhatsApp's 24-hour window, consent requirements, promotional rules — relaxes when the person contacted you first.
That is not a loophole. It is the system working as designed. So, as a design principle: programmes that give customers reasons to write first are structurally cheaper and more deliverable. Examples: "Reply 1 to confirm, 2 to reschedule"; a "Message us on WhatsApp" button on your checkout page; a keyword printed on a receipt.
The filtering nobody documents#
Operators' filters read message bodies. Certain words, link shapes and repetition patterns raise a score; above a threshold the message is dropped, often silently.
- Avoid shared link shorteners. A public shortener's domain is shared with every sender, including bad ones, and reputation attaches to the domain. Use a domain you control.
- Test with real content. A test that passes with placeholder text tells you nothing about the real message with the word "free" and a discount code.
Before a business message goes out
- Links on a domain you control
A public shortener's domain carries every sender's reputation, including bad ones
- Tested with the real content
Placeholder text tells you nothing about the word "free" and a discount code
- Assuming a dropped message will be reported
Above the filter's threshold it is often dropped silently
Throughput is not bandwidth#
A2P delivery is rate-limited per sender, and the limit depends on your registration and reputation, not your budget. Ten thousand messages do not leave at once; they queue, and the end of the queue can be well behind the start.
For time-sensitive messages — a flash sale, a service alert — stagger deliberately, put the time in the text ("until 6 pm today"), and never assume a broadcast is instant.
Where to go next#
- The five layers of a communications stack — the next step in this track.
- Sender registration around the world — what each market requires.
- 10DLC without tears — the US registration process in practice.
Sources
Every claim worth checking, with somewhere to check it.