feat: add more context about dtmf tool

Author: stevenbdf

fern/tools/default-tools.mdx

@@ -86,6 +86,16 @@ This function is provided when `dtmf` is included in the assistant's list of ava
 }
 ```
 
+<Note>
+There are three methods for sending DTMF in a phone call:
+
+1. **In-band DTMF**: DTMF tones are transmitted as part of the regular audio stream. This is the simplest method, but it can suffer from quality issues if the audio stream is compressed or degraded.
+2. **Out-of-band DTMF via RFC 2833**: This method sends DTMF tones separately from the audio stream, within RTP (Real-Time Protocol) packets. It's typically more reliable than in-band DTMF, particularly for VoIP applications where the audio stream might be compressed. RFC 2833 is the standard that initially defined this method. It is now replaced by RFC 4733 but this method is still referred by RFC 2833.
+3. **Out-of-band DTMF via SIP INFO messages**: In this approach, DTMF tones are sent as separate SIP INFO messages. While this can be more reliable than in-band DTMF, it's not as widely supported as the RFC 2833 method.
+
+As of writing, Vapi's DTMF tool uses in-band DTMF. Please note that this method may not work with certain IVRs. If you are running into this issue, the recommended approach is to have your assistant say the options out loud if available. For example, when an IVR says "Press 1 or say Sales for the Sales department," prefer having the assistant say "Sales."
+</Note>
+
 #### Send Text
 
 This function is provided when `sms` is included in the assistant’s list of available tool (see configuration options [here](/api-reference/assistants/create#request.body.model.openai.tools.sms)). The assistant can use this function to send SMS messages using a configured Twilio account.