The external call transfer – is the call transfer to a telephone line other than the company’s telephone line. The internal call transfer – the transfer of an incoming call to another addressee registered on the same telephone line.
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.