---
page_type: error
code: 13801
message: "13801: Refer not allowed on non-SIP call legs"
level: ERROR
product: Programmable Voice
---

## Description

The Refer verb is only allowed on SIP call legs, e.g. you can refer an incoming SIP call that originate from SIP infrastructure or an outbound call to a SIP destination. Calls to or from other destinations, such as a phone number or Twilio Client cannot be referred.

### Possible Causes

- Your application attempted to invoke the Refer verb on a PSTN or Twilio Client call leg.

### Possible Solutions

- For more information on understanding what does and does not constitute a SIP call leg, please see [Understanding SIP Call Legs](/docs/voice/twiml/refer#understanding-call-legs).
- While invoking Refer on a SIP leg is unique in that it removes Twilio from the call path, the effect of transferring a call to another party can be accomplished on all leg types by simply using the Dial verb to call the other party.