yes, to me, this is correct.
Be sure to use the TCH erlang, and not the SDCCH + TCH.
and then, you must know that during the TCH phase, there are few seconds during which the call is being connected and during which there is ringing. This delay is included in the TCH erlang, but is not being paid by the subscriber.
Therefore, I would remove 5% of the total erlang value. (9.05 – 5%)
(average call duration = 60 seconds, average waiting duration = 3 seconds)
I hope i’m being clear and logic 🙂