[Asterisk-Users] Network design question

[Asterisk]

Hello!

My business is a wireless ISP.  I would like to offer voice to several 
business customers.  I have a * server, but still need some hardware cards 
for it.
I would like to provide individualized billing to these customers for their 
usage.  Most of these customers have Nortel PBX's.   Could I use a Cisco 
ATA  or a similar manufacturer to provide 1-2 line service?  I would like 
to get this set up correctly the first time.  What would you suggest or has 
someone already done this that could explain better?

Thanks,

Bill