Yes, it is. It should work that way by default. However, it’s very clunky - you have to type dial and answer commands at the asterisk console command line.
Yes. There are a couple of contention issues, though, between the softphone and asterisk. You can disable asterisk’s use of the sound card by setting the following in modules.conf :
noload => chan_alsa.so
noload => chan_oss.so
And you will also have to change to SIP (or IAX) port that the softphone listens on - as you can’t have both asterisk and the softphone listening on the same port. You will have to configure asterisk to use the correct port when talking to the softphone.
You can use IAX - it’s probably the best way. But you could also use SIP if you wanted or needed to.