asterisk users - Sep 2005 - How Separate a few channels from the others on a PRI

Okay, here is the background.  I have a PRI with 15 active channels on 
it.  I originally setup all of them in group=1 and all outgoing and 
incoming calls used this group.  The phone number that I have associated 
with these channels ends with 750 and that is how I direct the calls.  
i.e. In my extensions.conf I have:

exten => 750,1,Dial(SIP/120,20)

All this works fine.  Now I have the need to separate out three of the 
channels (13-15).  I am using the associated phone number ending in 767  
for this purpose.  I have currently changed the zapata.conf to look like 
this:

signalling=pri_cpe
switchtype=national
echocancel=yes
echocancelwhenbridged=yes
echotraining=yes
callerid=asreceived
group=1
context=default
channel=>1-12
group=2
context=fax
channel=>13-15

To my understanding this is supposed to separate channels 1-12 into 
group 1 and channels 13-15 into group 2.  If that is true, that's fine, 
but it doesn't help me with my current issue.  This is what I would 
like...   When a call comes into the phone number ending in 750 it uses 
up the 12 channels in the first group, and when a call comes into the 
phone number ending in 767 it uses the 3 channels in group 2 AND USES NO 
MORE CHANNELS, only those three.  The reason for all of this is that I 
have faxing through asterisk working and want those last three channels 
used for faxing but I do not want the fax lines eating up all my 
channels and leaving none for voice calls.  Please help.  Thanks.
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