So what do you want to happen to the caller? Most want to go to a voicemail. If so, set up FWD Busy/NA to the voicemail unit, or set up IRNA to the voicemail unit. Further details depends on what voicemail you're using (if any).
If you're not using a voicemail, and you just want to ring the extension forever, then you'll need to transfer to a queue, which takes a little bit of coercing sometimes, depending on the specifics you want.
Or, if you mean callers are returning to the PBX Operator, instead of the extension that forwarded the call, then you'll need to clear-out the extension/extensions Transfer Recall Destination.