diff --git a/fern/chat/sms-chat.mdx b/fern/chat/sms-chat.mdx
index 05dfc937c..b1bbdd07d 100644
--- a/fern/chat/sms-chat.mdx
+++ b/fern/chat/sms-chat.mdx
@@ -22,8 +22,13 @@ 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)
+* On pay-as-you-go plans, a **saved payment method** on your organization (see the note below)
 * Basic understanding of phone number management
 
+<Warning>
+SMS replies are generated by the Chat API and follow its billing requirements. On a pay-as-you-go plan, your organization needs a saved payment method, and during its first 30 days one completed payment, before Vapi replies to a text. Without them the inbound text is received and a session is created, but no reply is sent and the Dashboard shows no error. Voice calls are not affected. See [Chat troubleshooting](/chat/quickstart#troubleshooting) and [Manage billing and credits](/billing/manage-billing-and-credits).
+</Warning>
+
 ---
 
 ## Setup Steps
diff --git a/fern/phone-numbers/inbound-sms.mdx b/fern/phone-numbers/inbound-sms.mdx
index ba0610d82..424ecb48e 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
+- On pay-as-you-go plans, a **saved payment method** on your organization, and one completed payment during the organization's first 30 days. Without it, inbound texts are received and a session is created, but no reply is sent and no error is shown. See [Chat troubleshooting](/chat/quickstart#troubleshooting).
 
 ## Set up from the Dashboard