diff --git a/README.md b/README.md
index 262c00a..3283a6e 100644
--- a/README.md
+++ b/README.md
@@ -16,6 +16,12 @@ Your SIP PBX should be comaptible with `L16@48000` or `OPUS@48000` voice codec.
 4. Set SIP server settings in `settings.ini`
 5. Run `tg2sip`
 
+SIP->Telegram calls can be done using 3 extension types:
+
+1. `tg#[\s\d]+` for calls by username
+2. `\+[\d]+` for calls by phone number
+3. `[\d]+` for calls by telegram ID. Only known IDs allowed by telegram API.
+
 ## Donate
 
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