Is there any way to answer the external line that is ringing to another phone?
e.g when the secretary is outside the office and her phone is ringing is there any shortcut to pick up the line from my phone without having to go to pick the phone physically? 
yes, it’s called Call Pickup. the implementation depends on the technology you’re using, and the version of Asterisk you’re running.
best place to start is the wiki … voip-info.org and search for “Call Pickup”.
i use the Asterisk Pickup() application here … work fine. it’s in the 1.2 tarball and later builds.