I don't think you can do that from Elastix itself, you will have to drop into the "dreaded bash shell".
and edit /etc/zaptel.conf
This process is well documented but basically it should look something like:-
span=1,1,0,esf,b8zs
# Global data
e&m=1-24
loadzone = us
defaultzone = us
and change the master timing,loadzone,defaultzone and signaling to suit your deployment.
if you have any other zap type hardware be careful with the order they are loaded in as it will change the order of the channels that asterisk will be presented with from the zaptel module.