asterisk users - Jan 2005 - how to manage Digium TDM04B outgoing calls correctly

I'm installing my first Asterisk server. I have a TDM04B card installed 
in my asterisk server (4x FXO ports). I have 5 Cisco IP phone 7960 
working fine on asterisk using SIP. My configuration to receive call is 
working as expected meaning anyone calling on one of the 4 FXO ports is 
answer by asterisk and asked to enter the extension of the person to 
reach and then it is transfer on the right Cisco phone. My question 
concern outgoing calls. How can I configure my extensions.conf to get a 
PSTN line on my TDM04B card in the following order : first trying on the 
channel 4 then if 4 is busy then switch to 3 if 3 is busy then switch to 
2 and if 2 is busy then say there's no more line available. I don't want
to dial on the first channel as it's my main number and all calls are 
received by that channel if the line isn't busy. I don't have to manage 
it for incoming calls (it can't be done anyway) as it's done not on my 
side but on the side of my provider (with the callforward on busy thing) 
but for outgoing calls the asterisk server must manage it.

The only example I saw for outgoing calls were using only one FXO card 
(X100P I think) so I'm sure it's not that hard but I have no clue how to
type it in asterisk.

That brings me another question. I'm currently testing with one TDM04B 
card but once I'll switch our current PBX to Asterisk I'll be using over
30 Cisco 7960 phones and we have 12 PSTN lines so I was considering 
buying 2 more TDM04B cards and one TDM40B (to have 4 FXS ports to 
connect Fax machines) but with all the problems people seems to have 
with these cards I'm not sure anymore.

Here's some specs of my server if that help answer the question :

Motherboard Tyan S2875ANRF with dual opteron processors, 1GB RAM, dual 
Western Digital Raptor 74GB 10000rpm running Fedora Core 3 AMD 64bits 
version. The motherboard comes with 5 PCI and 1 AGP slots so I would 
fill up all the slots with the cards. The server is an Antec 3U case 
with a 550W Power Supply.

So if you think the server can handle 5 TDM400P cards let me know. If 
not what FXOs device I should purchased? I don't want to switch to PRI 
for now (I know it would be easier to setup that way) as will be moving 
to a new building in a year or so and don't want to invest on that for 
now (as there's only one PRI provider where we are currently)

Any help would be appreciate :-)

Thanks

Martin Roy

> So if you think the server can handle 5 TDM400P cards let me know.
I've done an installation with 5 TDM400P cards - 4 PSTN lines and 12
analog phones.

There are no outstanding issues that havent been solved by tweaking a
particular config option (e.g echo, callprogress issues etc...).


Gerald.