--- title: "A2P, P2A, P2P: why the direction of a message decides everything about it" summary: "The same 160 characters are priced, routed, filtered and regulated differently depending on whether a person or a program pressed send — and on who wrote first." canonical: https://cpaas.co/en/learn/what-a2p-means kind: explainer depth: foundation origin: original published: 2026-10-03T16:26:18.899Z updated: 2026-10-14T16:26:19.416Z topics: ["A2P", "Messaging", "Fundamentals"] language: en --- # A2P, P2A, P2P: why the direction of a message decides everything about it ## 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. ```diagram:compare 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 > The inbound direction is the cheapest and least restricted, because permission given by action is the strongest permission there is. ``` ## 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. ```diagram:flow # How business traffic came to be separated Built for people texting people | Priced that way: bundles, flat rates, unlimited plans. Businesses discover the price | Anyone can be reached for the price of a personal text. Abuse at scale | A channel with no gatekeeper becomes a channel nobody reads. The 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](/en/learn/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. ```diagram:scale # The volume each number type is good for Low volume | High volume Long code | 2 | Two-way conversations, low to moderate volume Toll-free number | 5 | Two-way at moderate volume, with its own verification Short code | 9 | High volume and fast delivery — expensive and slow to obtain > The alphanumeric sender ID is not on this line: it is chosen for recognition, and it is one-way. ``` ## 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. ```diagram:checklist # 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](/en/learn/the-five-layers) — the next step in this track. - [Sender registration around the world](/en/learn/sender-registration-around-the-world) — what each market requires. - [10DLC without tears](/en/learn/10dlc-without-tears) — the US registration process in practice. ## Sources [^1]: [Ten-Digit Long Code (10DLC) Registration Guidelines](https://learn.microsoft.com/en-us/azure/communication-services/concepts/sms/ten-digit-long-code-guidelines) — Microsoft Learn [^2]: [Deadline for mandatory registration with Singapore SMS Sender ID Registry expires on 31 Jan 2023](https://cms.law/en/sgp/legal-updates/deadline-for-mandatory-registration-with-singapore-sms-sender-id-registry-expires-on-31-jan-2023) — CMS, January 2023 --- Published by the company behind five products in this category. 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