First of all, i am most probably younger in this field than a few other posters on this board, so excuse me -and please explain me- if i am wrong.
You are talking about holding time and call time.
holding time is about signalling (call setup) and should be around 1 or 2 seconds in a GSM network.
Call time is about the call in itself, the conversation between people and can last for a while (70 seconds in average for the same GSM network).
Are you doing (on purpose or not) some comparison between SDCCH and TCH traffic?