diff --git a/services/message_service.py b/services/message_service.py
index 889835b..83b3b89 100644
--- a/services/message_service.py
+++ b/services/message_service.py
@@ -51,7 +51,11 @@ async def send(db: Session, req: SendMessageRequest) -> Message:
     else:
         recipient_id = req.recipient_phone
         contact = contact_service.get_or_create_by_phone(db, recipient_id)
-        channel_conv_id = recipient_id
+        # WhatsApp: channel_conv_id immer als "nummer@c.us" normalisieren
+        if req.channel == "whatsapp":
+            channel_conv_id = recipient_id.replace("@c.us", "").lstrip("+") + "@c.us"
+        else:
+            channel_conv_id = recipient_id
 
     conv = conversation_service.get_or_create(
         db, req.channel, channel_conv_id, contact