Here is a little info just to make sure your calls and set up meet the requirements for Call Holding;
Call transfer and screening settings control how Cisco Unity handles your incoming calls from callers who did not dial you directly.
Transfer and screening settings apply only when calls are routed to you from the main greeting recorded for your organization, directory assistance, or a similar automated call handler that your Cisco Unity administrator set up to assist callers who dial the main phone number for your organization. This means that when an outside caller or another subscriber dials your extension to reach you by phone, your Cisco Unity transfer and screening settings do not apply.
i'm transferring calls from call handler on unity,so the call transfer should working,actually the call flow is as the following :
caller dial call handler number 2222,the caller get greeting message after the greeting the caller will be transferred automatically to specific subscriber ,and this subscriber is configured with supervisor transfer so that the caller stay on hold,but the caller going directly to the voice mail instead of being put on hold.