Tcas in my experience is turned on with the txponder selection from standby to on which usually happens entering the runway till exiting the runway on the other end. I believe some older aircraft have a dedicated switch for activation of TCAS seperate from others avionics. TCAS RA on many aircraft reverts to TA capability only during critical phases of flight, 400 - 1000AGL. The voice capability is also supressed during this regime as to not distract the crew at low altitudes. These are my experiences, other models may operate in a different manner.