You are talking about a recall feature, where, upon reaching a busy signal, you can dial a code and be rung back when the line is free?
I saw this submitted as a suggestion, a while back, but, from what I've seen so far, it doesn't look to be implemented in version 11.
http://3cx.ideascale.com/a/dtd/Ringback-when-internal-extension-busy/353778-9854