---
title: What does UNKNOWN_SUB (Unknown User) mean?
description: "The **recipient number is not recognized** as a valid subscriber on the network - for example the number does not exist, is not in service, or has been disconnected. Your message could not be delivered. **What you can do:** Verify the number (country…"
url: "https://www.smsgatewaycenter.com/blog/kb/what-does-unknown_sub-unknown-user-mean/"
updated: "2026-04-27"
category: "Bulk SMS / Errors & Error Codes"
---

# What does UNKNOWN_SUB (Unknown User) mean?

The **recipient number is not recognized** as a valid subscriber on the network - for example the number does not exist, is not in service, or has been disconnected. Your message could not be delivered.

**What you can do:** Verify the number (country code, digit count). Remove or update invalid or disconnected numbers from your list.

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