---
"@context": https://schema.org
"@type": TechArticle
"@id": https://www.twilio.com/docs/api/errors/13801#article
headline: "13801: Refer not allowed on non-SIP call legs"
url: https://www.twilio.com/docs/api/errors/13801
inLanguage: en
dateModified: 2026-03-11T18:38:39.000Z
author:
  "@type": Organization
  name: Twilio Developer Education Team
publisher:
  "@type": Organization
  name: Twilio
---

# 13801: Refer not allowed on non-SIP call legs

Log Type: APPLICATION

Log Level: ERROR

## 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.