Is it possible

Asterisk Asterisk Support
1

Is it possible to have a setup where a Linux server, acting as a gateway,
with a voice modem (and eventually a sound card) and asterisk on it can
redirect call incoming, on the basis of an extension entered after the
modem has pick up the line.

if no. Why?

2

yes, sounds like you are defining an IVR, asterisk will do that just fine.