How can I use external modem for voice calls on Android tablet? My tablet doesn't support calling/messaging function by default and also it doesn't have an app for that by default. 

Is it possible? In my case I use Micromax Infinity P275 tablet and Huawei E1732 modem. ICS is my OS. I can connect to the internet with that modem.
Root your phone, and download PPP Widget from Google Play.