I found in some other manufacture IP phones support this feature (change in response code) in their configuration file as  "features.normal_refuse_code = 603" , i believe there might be something in Cisco phones configuration as well , else its good ...

I see in configuration under menu programmable Soft keys ,there is ringing key list, i.e. answer|1;ignore|2;ignoresilent|3; , i tried to add endcall instead of ignore , but it didn't work, any other keyword it supports?  that might change the behavio...