It is a device capturing and controling all the signals, which are being created by the whole system, especially the made and received phone call signals. The device is mostly used for connecting regular phone system with the Internet and backwards.
Asterisk cards have a different number of ports, due to the variety of needs for different applications, manufacturers try to address them. For beginners, one can request for Asterisk card with single port configured with 1 FXO module. At first, you receive the option of accept incoming calls, afterwards you can route them anywhere within your IP network. It’s only in case of a live call, and than your line is busy because you have a single FXO to handle that call.