diff --git a/README.md b/README.md
index 85bc066..8a4b1d2 100644
--- a/README.md
+++ b/README.md
@@ -180,6 +180,8 @@ It is best effort: characters with no safe equivalent pass through untouched. Ch
 
 ### Phone number helpers
 
+textbee does not require international format. Local numbers are sent as written and reach recipients in the same country as the sending SIM. Short codes are sent as written too. They work only in the sending SIM's country, and some work only on one carrier. International format works for every recipient, so use these helpers when you send to more than one country or match replies to stored numbers.
+
 ```js
 import { isValidE164, normalizePhoneNumber } from '@textbee/sdk'
 
diff --git a/src/types.ts b/src/types.ts
index c3b560d..79a5025 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -18,7 +18,12 @@ export interface SendSmsRequest {
   /** The message body. */
   message: string
 
-  /** Phone numbers to send to, in international format. */
+  /**
+   * Phone numbers or short codes. International format (`+12025550123`)
+   * reaches any country. Local numbers reach recipients in the sending SIM's
+   * country. Short codes work only in that country, and some only on one
+   * carrier. Values with letters are rejected with 400.
+   */
   recipients: string[]
 
   /**