Atis,
You are running into a timeout issue. When you start the transfer process, you essentially seizing a new channel and placing a call. There are timeout settings in CUCM that are related to inter-digit timeouts. Namely:
Off-Hook to First Digit Timer. This is the max time (in ms) that CUCM will wait for the first digit to be presented after the off hook event. This is most likely the timer that is causing your woes. The transfer softkey triggers a new call and the CUCM is waiting for a digit. The timeout is not adequate enough for your workflow (searching via corp directory). The default value is 15000 (15s) and can be adjusted. It is system wide.
T302 Timer. This timer applies to inter-digit timeout AFTER the first digit is pressed. It is also 15000ms by default.
The two timers do not need to be the same.
HTH.
-Bill (http://ucguerrilla.com)
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