Is it possible to receive an incoming call while tethering on an Android phone? Is it possible to have a 3G tethering session and receive an audio call at the same time? Is this the function of the phone or the mobile OS or the carrier-network?
It depends:


You are on a CDMA network (Verizon, Sprint) you can't call and use Internet at the same time (exepect if you use Internet over Wifi) - This is limitation of CDMA only networks
You are on a GSM network (AT&amp;T for US and almost all other operator worlwide excluding Asia). In this case if you have 3G connectivity you should be able to use 3G and call at the same time, if connectivity not 3G but only GRPS/EDGE you can't have a call and internet at the same time (because GPRS/EDGE use voice channel)