Hi Julio,
For CME :)
On-Hold Indicator
On-hold indicator is an optional feature that generates a ring burst on idle IP phones that have placed a call on hold. An option is available to generate call-waiting beeps for occupied phones that have placed calls on hold. This feature is disabled by default.
LED color display for hold state, also known as I-Hold, is supported in Cisco Unified CME 4.0(2) and later versions. The I-Hold feature provides a visual indicator for distinguishing a local hold from a remote hold on shared lines on supported phones, such as the Cisco Unified IP Phone 7931G. This feature requires no additional configuration.
SCCP: Enabling On-Hold Indicator
The Call Hold feature is available by default. To define an audible indicator as a reminder that a call is waiting on hold, perform the following steps.
SUMMARY STEPS
1. enable
2. configure terminal
3. ephone-dn dn-tag [dual-line]
4. hold-alert timeout {idle | originator | shared}
5. end
From this good doc;
http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmering.html#wp1013688
For CCM :)
The feature you want is called "Hold Reversion" This feature is available on CCM 4.2/6.x
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.
From this CCM 4.2.3 doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a008070e55c.html
Hope this helps!
Rob
