Line &1 is busy

What causes this issue?

The extension &V1& is currently busy, therefore calls cannot be
initiated nor other calls transferred to this extension.
More information about the external software component:
&V3&

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Procedure for System Administrators

The error can occur in 2 different places:
1. In the connected gateway
2. In the underlying CTI software
In this case, the error occurred in (&V4&).
The return code for the underlying CTI software is: &V2&

Error message extract from SAP system. Copyright SAP SE.