asterisk users - Jul 2005 - How do you dial an alternate line on busy with several multi-line phones?

Situation:

Dialplan requires several phones be rung on an incoming call, some of
which have multiple line presentations. If Line 1 on a phone is busy,
Line 2 should be rung instead.

Problem: Normal Dial() syntax only rings ALL extensions which we do not
want since we only want the second line on a given phone to ring if the
lower number line is busy.

More Detailed example:

3 phones with 2 lines each. When a call comes in and nobody is on the
phone we want to ring Line 1 of each phone only. This is easyly done as
follows:

Dial(SIP/Phone1ex1&SIP/Phone2ex1&SIP/Phone3ex1)

But what if line 1 of phone 1 is busy? How do you get it to roll over to
ex2?

This does NOT work because it rings all extensions:

Dial(SIP/Phone1ex1&SIP/Phone2ex1&SIP/Phone3ex1&SIP/Phone1ex2&SIP/Phone2ex2&SIP/Phone3ex2)

ChanIsAvail also does NOT work because it returns that the channel is
available even if it is busy and ${AVAILSTATUS} is always 0.

A natural idea would be ChanIsBusy(SIP/Phone1ex1&SIP/Phone1ex2) but no
such command exists.

Anyone know how to accomplish what I'm after?

-- 
John Lange