diff --git a/README.md b/README.md
index 106ed2d..118e57b 100644
--- a/README.md
+++ b/README.md
@@ -215,6 +215,8 @@ 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.
+
 ```python
 from textbee import is_valid_e164, normalize_phone_number
 
diff --git a/src/textbee/_client.py b/src/textbee/_client.py
index fb1df56..fbbbc0b 100644
--- a/src/textbee/_client.py
+++ b/src/textbee/_client.py
@@ -89,7 +89,11 @@ def send_sms(
     ) -> SendSmsResponse:
         """Send an SMS to one or more recipients.
 
-        ``recipients`` are phone numbers in international format.
+        ``recipients`` are 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.
 
         ``device_id`` picks the sending phone. Get ids from ``get_devices()``.
         Omit it and textbee picks the sender: your default device first,