https://www.ptcommerce.net/sipml5/call.htm?svn=251#
Receiving faxes with Asterisk, iaxmodem and HylaFax with dynamic eMail recipients
http://support.t38fax.com/support/solutions/articles/19000008323-freepbx
http://cawanblog.blogspot.pt/2010/01/how-to-downgrade-siemens-s450-sip.html
https://www.voip-info.org/wiki/view/Asterisk+firewall+rules
https://wiki.freepbx.org/display/FPG/IVR+Example+Configurations
https://wiki.freepbx.org/display/FPG/IVR+Module+User+Guide
https://github.com/bg111/asterisk-chan-dongle
https://jtanx.github.io/2016/02/24/using-asterisk-to-route-calls-through-mobile/#configuring-raspbx
Asterisk connection with Mobile phone using Chan_Mobile
http://www.dslreports.com/forum/r20475110-How-To-Add-a-Cellular-Trunk-to-Your-VoIP-System