does anyone has an idea how to communicate using my pc speaker and headphone when i make a call or receive a call from my gsm modem?
Do you mean with the headset still connected to the PC, so that the audio is routed through the PC?
Or do you want to connect the headset direct to the modem?
voice call is routed from gsm modem through pc. any idea?
Is it?
How?
You will need some software on your PC to route the signal from the headset mic to the GSM modem’s input, and from the GSM modem’s output to the headset earpiece(s).
I’m not sure that all soundcards can do this; you might need two - one connected to the headset, and one to the modem… 
(a USB headset effectively has its own buit-in soundcard)