An alphanumeric sender ID is a name in the from field of a text message. Instead of a number, the handset shows a short run of characters that you chose, which is usually your brand.
Two things about it surprise people, and both come from the same place: the field it travels in. It is small, and it is one-directional.
Why is an alphanumeric sender ID eleven characters?
Because of how much room the SMS address field has left after its own headers.
The specification that defines the field is 3GPP TS 23.040, published by ETSI. Its address field has three parts, and the whole thing is bounded:
Within the Address-Value field, either a semi-octet or an alphanumeric representation applies. The maximum length of the full address field (Address-Length, Type-of-Address and Address-Value) is 12 octets.
An alphanumeric address is one of the type-of-number values the same clause lists: "Alphanumeric, (coded according to 3GPP TS 23.038 [9] GSM 7-bit default alphabet)". And the clause above it says what that packing means:
A field which uses alphanumeric representation shall consist of a number of 7-bit characters represented as the default alphabet defined in 3GPP TS 23.038 [9].
Both quotations are from ETSI TS 123 040 V17.2.0, clauses 9.1.2.4 and 9.1.2.5.
Put the two together and the ceiling falls out. Twelve octets, minus one for the length and one for the type, leaves ten octets of address value. Ten octets is 80 bits, and at 7 bits per character that is 11 whole characters with 3 bits spare. That is the eleven everyone quotes, and it is a property of the network rather than a policy any provider chose.
Bird enforces the same number. The sender string is 1 to 11 letters, digits, spaces, dashes or underscores, at least one of them a letter, and no separator at either end. Those are the constraints on the sender field of sms senders create, and a string that breaks one is refused when you claim it rather than when you send.
Can anyone reply to an alphanumeric sender?
No, and not as a matter of policy. There is nothing to reply to.
A reply is a message addressed to the sender of the one that arrived. A number is an address; a name is not. Bird's own model says the same thing in its data: an alphanumeric sender's country code is null, "the country of the number this sender sends from", and its number_id is "present only when source_kind is number or shared_number; an alphanumeric sender has no number behind it".
So an alphanumeric sender is a send-only identity. If the message you are sending invites an answer, a stop keyword included, it needs a sender a handset can address. One-way and two-way SMS covers what changes when you need the reply.
That has a consequence beyond the product. A regulated destination that gives recipients a right to opt out has to reckon with a sender they cannot text back, and at least one regime answers it explicitly: where the protocol does not allow reply texts, each message has to disclose that clearly and offer another way to revoke. What is the TCPA has that provision. Plan the alternative before you send from a name, rather than after someone tries to stop you.
What does an alphanumeric sender cost, and when?
Claiming the string is free. Registering it to send somewhere may not be.
Claiming records the string against your workspace so nobody else can take it. The charge, where there is one, attaches to a country registration: a priced alphanumeric sender carries a one-off setup fee and a recurring monthly charge wherever the rate card prices the country. The moment it bills depends on the country's own answer. Where a country approves the registration immediately, the fee lands on that call; where a country reviews it, the registration is created unbilled and charged if the review approves it. Registering a number is free either way.
Neither the registration call nor the requirements read quotes a price, so ask whoever owns the account before you register a priced sender. SMS pricing publishes the rates.
Where does an alphanumeric sender work?
That is a per-country answer, and it moves, so it lives on the page that reads it live rather than in this one.
Three separate things have to be true before an alphanumeric sender reaches a handset in a given country: the country has to carry the alphanumeric sender type at all, you have to satisfy whatever registration it asks for, and the destination has to be switched on for your workspace. The first two are published per country on SMS destinations, alongside the other sender types that country carries. The third is your own setting, and a send to a country you have not enabled comes back as E12020 whatever your sender is.
You can ask the same question about one sender from a terminal, which is the version worth wiring into a check:
bird sms senders requirements <sender-id> --country-code NLEach row is one country this sender can reach, and it says whether that country needs a registration first, which programme runs it, and where the sender stands with it. Drop the flag to get every country relevant to the sender; the command reference carries the rest. SMS sender IDs walks the same read in the dashboard.
What should I do before I claim one?
Four things, in this order.
- Check the destinations you actually send to on SMS destinations, because a country that does not carry the alphanumeric type will not carry it for you either.
- Pick a string that survives the cap. Eleven characters is not much, and a name truncated by the network is worse than a shorter name you chose.
- Decide whether you need replies. If you do, an alphanumeric sender is the wrong identity and the sender-type comparison has the alternatives.
- Find out what registration will cost before you register, since claiming is the free half and registering is not.