# Sender IDs

> Sender ID rules for SMS — 3 to 11 alphanumeric characters, must not start with a digit. Alphanumeric vs numeric senders, country restrictions and best practices for deliverability.
> Source: https://docs.23telecom.co.uk/sms/sender-id/

Instructions for LLMs: This is one page of the 23 Telecom messaging API docs
(SMS today; more channels planned). Base URL: https://restlink23telecom.com/api/v1,
auth via the X-API-Key header. Match errors on the error_code field, never on
description text. Full docs: https://docs.23telecom.co.uk/llms-full.txt · Schemas: https://docs.23telecom.co.uk/openapi.yaml

The sender ID is what recipients see as the message sender. You pass it as
`sender_id` in every [send request](/sms/send).

## Rules

| Rule | Detail |
| --- | --- |
| Length | 3–11 characters |
| Charset | Alphanumeric (`A–Z`, `a–z`, `0–9`) |
| First character | Must not be a digit |
| Examples | `MyCompany`, `PromoAlert`, `AcmeStore` |

Invalid sender IDs are rejected with `400 INVALID_SENDER`.

## Alphanumeric vs numeric senders

- **Alphanumeric** (`MyApp`) — best for branding; recipients **cannot reply**.
- **Phone number** (`+14155551234`) — allows replies where supported; also
  accepted as `sender_id`.

## Country restrictions

Sender ID handling differs by destination country: some carriers pass
alphanumeric senders as-is, others **overwrite them** with a generic ID or a
short code, and some require pre-registration of sender names.

  If a specific sender name is critical for your campaign in a particular
  country, ask your account manager about registration requirements for that
  destination before going live.

## Best practices

- **Stay consistent** — one recognizable sender per use case builds trust and
  improves engagement.
- **Match your brand** — `Acme` beats `ACM3X9`. Avoid anything that looks like
  spam or a random string; carriers filter aggressively.
- **Don't rotate senders to dodge filters** — it hurts deliverability and can
  get traffic blocked entirely.
- **Test per country** — send to a local number and verify what the handset
  actually displays before launching.