Transfers acting like transfer back on busy, but not using that

[korazy]

3CX Standard v16.0.1078
All phones are Grandstream GXP1630

Settings, General, Transfer Back on Busy
Dial Code: <blank>
Timeout: 2

For the extension being transferred to:
Status: Available
If I do not answer calls within: 15 seconds
Forwarding Rules are all set to Forward to Voicemail.

Reception answers an external call,
presses 'Transfer' button on phone,
dials destination extension,
presses the 'Send' button on phone

Sometimes, not always, the transfer will ring the destination extension one time, then bounce back to reception. Acting like the 'Transfer Back on Busy' which we are not using.
How do I troubleshoot this or what could be the cause?

 

[VictorYourITGroup]

Hi Korazy,
Use the Activity Log off the main console to see what is happening.
https://www.3cx.com/blog/docs/3cx-server-activity-log/
https://www.3cx.com/blog/docs/server-activity-log-example-1/
The blog entries are a little old, so screens have changed, but the information will help you with troubleshooting.
Good Luck!