Record only answered calls and put dialed number in file name of the recording

I want to record all outgoing calls and I selected always record on all extensions.
How do I tell Trixbox, or Asterisk, to record only answered call, since it is recording everything, answered and not answered calls, so I have a lot of ringing without answer recorded ?
And another thing is how do I put dialed number in file name of the recording ?

Anybody any idea ?

Any part of the problem ?

Directions ... ?