Core Issue
This issue can occur if the Ring Subscriber's Extension option is not enabled
Resolution
To overcome this problem, enable the Ring Subscriber's Extension option .
To transfer a call from a call handler to an operator's IP phone when callers press 0, you need to select the Send Caller option Yes, ring subscriber's extension. Select the operator subscriber. In the operator subscriber, under the Call Transfer option in Subscribers, select Yes, ring message recipient's extension.
To prevent this, the default subscriber template must be modified. To do this, go to Subscriber > Subscriber Template > Call Transfer.
Under Transfer incoming calls to subscriber's phone?, check one of these options:
Save the setting and all subscribers created with the template. With the proper configuration, calls are transferred to the subscriber's phone instead of going directly into voice mail.
If the subscriber has already been created, you can modify this setting for an individual subscriber by going to Subscriber > Subscribers > Call Transfer and checking one of these options:
Yes, ring subscriber's extension
Yes, ring subscriber at this number (enter extension to ring)
You can use the BulkEdit utility to modify several subscribers at once.
Once the Call Transfer setting is modified, all calls are transferred to the subscriber's phone instead of going directly into voice mail.
For related information, refer to these documents: