I guess your Call Park display timer is very short. You may want to increase that timer. Default is 10 seconds. It is cluster-wide parameter.
Check the following
---------------------------------------
Setting the Service Parameters for Call Park
Cisco CallManager provides two clusterwide service parameters for call park: Call Park Display Timer and Call Park Reversion Timer. Each service parameter includes a default and requires no special configuration.
â¢Call Park Display Timer-Default specifies 10 seconds. This parameter determines how long a call park number displays on the phone that parked the call. Set this timer for each server in a cluster that has the Cisco CallManager service and call park configured.
â¢Call Park Reversion Timer-Default specifies 60 seconds. This parameter determines the time that a call remains parked. Set this timer for each server in a cluster that has the Cisco CallManager service and call park configured. When this timer expires, the parked call returns to the device that parked the call.
--------------------------------------------------------------------------------
Note To set the timers, choose System > Service Parameters and update the Call Park Display Timer and the Call Park Reversion Timer fields in the Clusterwide Parameters (Feature-General) pane.