SIM

SIM-based sending means texting from a phone number tied to a physical mobile SIM — the kind of number a handset uses. It sits on the routes carriers built for person-to-person conversation, which is why it behaves differently from registered application-to-person (A2P) sending.

What SIM-based sending is#

A SIM number is provisioned by a mobile carrier for a device. Messages sent from it travel the same person-to-person routes as everyday texting between two people. That makes SIM-based sending feel simple: little registration, a local number, and conversational back-and-forth.

That simplicity is also its limit. Person-to-person routes assume a human sending at human pace to people who expect to hear from them.

How it differs from registered A2P#

Registered A2P sending — such as A2P 10DLC on a standard 10-digit number — starts from the opposite premise: software is doing the sending, so the sender registers who they are (brand) and how they message (campaign) with the carrier registry. Carriers then know whose traffic they are carrying and why.

SIM-based sending carries no such registration. When outreach software drives messages through SIM numbers at volume, the traffic is application-to-person in substance even though it travels person-to-person routes — and carriers evaluate it accordingly.

Why carriers treat it differently#

Carriers protect person-to-person routes for actual conversation. Traffic that looks automated — high volume, repeated identical wording, many unknown recipients — does not match how people text each other, and carrier policies generally expect that kind of application-driven traffic to be registered. Unregistered application-style traffic on SIM numbers can be filtered, and the numbers themselves can face carrier action.

Registered A2P flips that dynamic: because the sender and use case are known, carriers can deliver wanted messages instead of guessing at intent.

SIM and Landing Zone#

Registered A2P messaging is the supported path in Landing Zone. You register your brand and campaign with the carrier registry through Twilio, then enter your Brand ID and Campaign ID in Settings → Trust Center, which tracks your registration status — see the A2P 10DLC setup guide.

For a side-by-side look at SIM, A2P 10DLC, and toll-free — including when each tends to fit — see SIM vs 10DLC vs toll-free.

Your responsibilities#

No sending method makes a particular message lawful, and none replaces permission. Whichever route your messages travel, have consent to contact each person, keep your sender recognizable, and honor opt-outs immediately. See Consent and contact permissions and Carrier requirements.