In Two TRX means TWO ARFCNs/GSM Freqs.
One Freq is Equal to = 8 TS
Tw Freqs so 16 Time Slot.
One Timeslot for BCCH.
One Timeslot for SDDCH*8.
One Timeslot can Handle 8 SDCCH.
So remaining 14 Can be used as TCHs.
I heard that there is a combined methode.But Idle is the above 0ne.
In the above set up at the same time you can do 8 Call attempts only.
But Can serve 14 Calls easily.
Rgds
Sunil