You can use asterisk to change your out going number, but you cant recieve calls from it. I use X-Lite sip phone, this allows you to put in your own configs. Set up an account with voipjet. Visit
www.voipjet.com to signup to their outgoing service in order to
make outgoing calls.
(I do not know of any incomming service to recommend)
once you sign up for VOIPJET , carefully read their bullshit settings and
enter them where they should go in asterisk. You will need to download X-lite software sip-phone to a computer networked
to your asterisk server. Your asterisk server MUST MUST MUST be available to the internet through
DMZ . You can configure DMZ in your internet router by logging into it
with a web browser and changing DMZ setting to match your asterisk servers
IP ADDRESS. make sure to enable DMZ.
Configure your sip-phone with your asterisk server's ip address
[192.168.0.2] , extension number [2000] and of course the password
[123456].
these settings will differ from what might actually be entered in your
sip.conf and extensions.conf files.
Also visit Visit
http://voxilla.com/ for wizards..... configurations and tips
asterisk related.
Good Luck!!!