Currently there are features which support quad rate calls in the system ie 4 simultaneous calls at one time slot.
in Ericsson : it’s known as VAMOS feature and in Nokia : it’s known as OSC feature.
Ex: 1 TRX=8 tsl
using combined mode we have 7 TCH TSL.
Total Voice calls supported by TRX= 7*4=28 calls simultaneously.