send SMS Using asp.net website?

There are modems available to send or recive SMS. These hardware will be connected to your PC through COM ports. Look at the AT Commands and that will tell how can you do low level communication with the device. Or you can buy a SDK from the Hardware manufacturer itself.

Serach Google:
Keywords:
1) gsm modems for sms
2) at commands for gsm

As already told you can use AT commands to write your own device communication layer (COM communication using Communication APIs) Or Purchase a SDK and Make simple function calls from your App. Talk to your boss not reveling you can write your own communication layer and purchase a hardware and SDK. Your job will go easy