diff --git a/fern/chat/sms-chat.mdx b/fern/chat/sms-chat.mdx
index 05dfc937c..c3489c194 100644
--- a/fern/chat/sms-chat.mdx
+++ b/fern/chat/sms-chat.mdx
@@ -23,6 +23,11 @@ SMS chat requires a **10DLC-approved Twilio number**. Only customers can initiat
 * A [Vapi account](https://dashboard.vapi.ai/) with an existing assistant
 * A **10DLC-approved Twilio phone number** (required for assistant responses)
 * Basic understanding of phone number management
+* A **saved payment method** for a pay-as-you-go organization
+
+<Warning>
+SMS replies use the Chat API and follow its billing requirements. Pay-as-you-go organizations need a saved payment method and, if they are less than 30 days old, one completed payment. Without these, Vapi receives the inbound SMS message and creates a session but does not send a reply or show an error in the Dashboard. Voice calls are not affected. See [Chat troubleshooting](/chat/quickstart#troubleshooting) and [Manage billing and credits](/billing/manage-billing-and-credits).
+</Warning>
 
 ---
 
diff --git a/fern/phone-numbers/inbound-sms.mdx b/fern/phone-numbers/inbound-sms.mdx
index ba0610d82..9d5aab679 100644
--- a/fern/phone-numbers/inbound-sms.mdx
+++ b/fern/phone-numbers/inbound-sms.mdx
@@ -23,6 +23,7 @@ When enabled, Vapi configures the Twilio Messaging webhook on your number so inb
 - **Twilio number in the US**
 - **SMS capability enabled** on that number (in Twilio and in Vapi)
 - **US-to-US** messaging only
+- **Pay-as-you-go billing:** Pay-as-you-go organizations need a saved payment method and, if they are less than 30 days old, one completed payment. Without these, Vapi receives the inbound SMS message and creates a session but does not send a reply or show an error. See [Chat troubleshooting](/chat/quickstart#troubleshooting).
 
 ## Set up from the Dashboard