Alphanumeric Sender ID Explained: Rules, Replies and Spoofing

An alphanumeric sender ID is a name, such as ACMESHOP, shown in place of a phone number as the sender of a text message. The business sending the message chooses it, it can be up to 11 characters long, and it's one-way: there's no number behind it, so you can't reply. Many countries allow named senders, the US and Canada don't, and a growing number require businesses to register their names first. Because the sender writes the name, it can also be faked, which is the most important thing to understand about it.

How an alphanumeric sender ID works

Every SMS carries an originating address, the "from" field. Usually that's a phone number, but the SMS standard also lets the sender mark the field as text. The receiving phone sees that and shows the text as the sender's name.

For a business, it's a per-message setting in the SMS provider's API. Instead of sending from +447700900123, the request says the message is from ACMESHOP.

The usual constraints:

• Up to 11 characters, because of how the field is encoded in the protocol.
• Letters, digits and spaces. Some providers allow a few punctuation marks as well.
• At least one letter. Providers usually insist on it, since an all-digit sender would look like a phone number.

Why you can't reply

When you reply to a text, your phone sends a new SMS to the original sender's number. With an alphanumeric sender ID there is no number, so your phone either disables the reply box or the reply simply fails.

That's why messages from named senders often say "do not reply" and point you to an app, a website or a separate phone number instead. It also means you can't text STOP to a name, so senders that use names for marketing need to offer another way to opt out.

Alphanumeric sender ID rules by country

Support varies more than most developers expect.

• US and Canada: not supported. Business texts come from short codes, toll-free numbers or registered 10-digit numbers. Depending on the provider, a named message sent there is either given a number as the sender or rejected.
• UK and much of Europe: widely supported and common for codes, delivery updates and alerts. The UK also has an SMS SenderID Protection Registry, run by the Mobile Ecosystem Forum, where organisations register their names so that participating networks and providers can block others from using them.
• Registration-first countries: some countries only deliver named messages from registered senders. Singapore requires organisations to register their sender IDs and labels unregistered ones as likely scams. India requires businesses to register their sender headers and message templates before sending.
• Rewritten senders: on some routes, the name is replaced with a number or a generic sender before it reaches the phone.

Before launching in a new country, ask your provider three things: is a named sender supported, does it need registering, and what happens to the name if it isn't.

Spoofing: the weak spot

The sender name is just text that the sender types. Nothing in the original SMS design checks that a message from "ACMESHOP" really came from Acme Shop. Scammers use this to send phishing texts under the names of banks, delivery companies and government agencies.

It gets worse on the phone itself. Messaging apps group texts into conversations by sender, so a spoofed message can land in the same thread as genuine messages from that organisation, right under real codes and alerts, where it looks completely trustworthy.

What helps:

• Registries, like the UK and Singapore schemes, block or label unregistered use of protected names on participating networks.
• Carrier filtering catches some spoofed traffic, especially from unusual routes.
• Habits close the rest of the gap. A familiar sender name proves nothing. Don't tap links in texts asking you to log in, pay or "verify" something; open the app or type the website address yourself.

For developers: using a named sender well

• Pick a name people recognise. Use the brand they signed up with, and keep it the same in every country that allows it.
• Put the brand in the message body too. The name may be replaced on some routes, and the body is what people actually read: "ACMESHOP: your code is 482913."
• Never ask people to reply to a message from a named sender. They can't.
• Register early where it's required. Approval can take longer than your launch plan assumes.
• Plan a fallback for countries without named senders, usually a number or a pool of numbers managed by your provider.
• Keep links out of code messages. A code text with no link is easier for people to tell apart from phishing.

To see how your sender actually shows up on a real network, send a test code to a public UK number on Temp SMS and check whether the page shows your name, a number or something else. Use a throwaway test account only: anyone can read what arrives and the numbers are retired over time, so never use them for banking, a main email or two-factor authentication.

FAQ

What does alphanumeric sender ID mean? A text message sender shown as a name made of letters, and optionally digits, instead of a phone number. The business sending the message sets it.

Can I reply to a text from a name instead of a number? No. There's no phone number to send the reply to, so contact the organisation through its app or website instead.

Are alphanumeric sender IDs allowed in the US? No. In the US and Canada, businesses text from short codes, toll-free numbers or registered 10-digit numbers.

Can scammers fake a sender name? Yes. Sender names can be spoofed, and a fake can even appear in the same thread as real messages. Registries reduce this in some countries, but a name alone never proves who sent a text.