Two lines handling

Hi to all,

in our organization many users have two lines, one desktop (main) and
one mobile (secondary); each one has different extension number, main
is e.g. 65, secondary is 1065.

The strategy should be that main extension ring, after timeout
secondary extension is rung. When one of two is busy, either for
incoming call or outbound generated call, a new incoming call should be
returned as busy.

Now, I get successfully working such ring strategy with the
follow-me, but on the busy issue I didn’t find yet any solution (I also
tested with a queue), when secondary extension is busy then main can
still receive incoming calls.

Thank you in advance.