Unfortunately, this feature is only available in Call Manager Express and not Call Manager. Im not sure about the newer versions of Call Manager, but I know for sure that 4.x doesnt have this feature. Therefore, if your users would like to be reminded of calls on hold, you may want to have them to use park as opposed to hold. Call Manager does have a hold duration timer that defaults to 360 minutes, and you can change this if you like.
The feature you want is called "Hold Reversion" This feature is available on CCM 4.2 Have a look;
The Hold Reversion feature alerts a phone user when a held call exceeds a configured time limit. When the held call duration exceeds the limit, Cisco Unified CallManager generates alerts, such as a ring or beep, at the phone to remind the user to handle the call. The held call becomes a reverted call when the hold duration exceeds the configured time limit.
The types of alerts that are generated at the phone for reverted calls depend on the capabilities of the phone device. Cisco Unified Call Manager provides the following alerts when the hold reversion feature activates, depending on the capabilities of the phone and the firmware release that is installed.
The phone rings once or beeps once.
The status line briefly displays "Hold Reversion" for the reverted call at the user phone.
The LED next to the line button flashes continuously on the phone handset, like other alerting operations.
A "wobbling" handset icon displays for a reverted call.
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