A few options are possible. You may choose the one based onyour convinience. The options are: -
a. Have your gatekeeper make the bandwidth choice based on the tech-prefix
b. If you have a set of E1-s dedicated for each customer, assign a trunk-group
for this set and make your dial-peer choice based on the trunk group number
of the incoming PSTN call.
c. You can use 'max-connection ..' under dial-peer voip configuration
to restrict the number of calls that can simultaneously be active using
a dial-peer. Based on the codec you have in your dialplan, this
command can be used to restrict the number of outgoing calls through
a given dialpeer