Asterisk

 iax configuration  ( not guaranteed ) 
 SIP configuration for voip phone 
 Working iax configuration, using vpn  Registration statement on one instance only ( Really vpn should not need a register statement ) 
 working iax config using nat firewall  Very similar to above but using nat instead of vpn.
 Configuring chan_mobile 
 sip2sis 
 Pattern matching  in the extensions.conf file.
https://shyju.wordpress.com/tag/dahdi-call-disconnect-tone-settings/ ( hangup issues )
cmterm-7975-sip.8-4-1.zip  is supposed to work with cisoc 7975G
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cuipph/7975g/6_1/english/administration/guide/7975GAdminGuide/7975cst.html#wp1088046
http://doxygen.asterisk.org/trunk/chan_dahdi.conf.html
 Calling via IAX 
http://www.electronics-tutorials.ws/filter/filter_4.html Band pass filter
http://www.electronics-tutorials.ws/filter/filter_7.html using op amp ( this looks good )
http://www.klaverstyn.com.au/david/wiki/index.php?title=Asterisk_for_Raspberry_Pi_on_Raspbian  Install asterisk manually.