A significant delay of up to ten seconds could occur during a call setup between two IP phones registered in the same Cisco CallManager cluster. This is primarily due to an overlap in the dialing plan. For example, an IP phone with the extension 555 and a call park range of 555xx could exist in the dial plan.
Cisco CallManager relies on the interdigit timeout to allow the user sufficient time to dial the number. If overlaps in the dialing plan cannot be avoided, you can shorten the interdigit timer value by performing these steps in Cisco CallManager:
Select Service > Service Parameters, and select server.
Find the TimerT302_msec* parameter, enter a value of 6000 and click Update. This sets the interdigit timer to six seconds.
Have the users test the phones and adjust as necessary.