To my knowledge, there is little you can do about the carrier choice when deploying your BES. The only more-or-less related parameter is the FQDN of the RIM "relay" to which your BES is connecting. But connectivity-wise with various wireless carriers the different RIM relays are supposed to be identical.
In other words, if RIM does not provide in the "backend" connectivity to your preferred carrier(s), that's it and your BES will not be able to contact the devices.
You can also verify this from the other end - if your preferred telco does sell a Blackberry Enterprise mobile subscription (bare/basic GPRS is NOT enough), it will work, and if it does not, it will not work.
Message Edited by Normunds on 06-03-2008 02:30 PM
