As in under Options -> Advanced Options -> TCP/IP ?
If you want to restrict all usage to BIS , then the APN settings should be disabled [first tick box off]
If you have apps that don't use BIS, then MTN and Vodacom's apn normally is "internet" [no username or password] . Not sure what "blackberry.net" does.
Anyway, i disable my APN settings and only turn it on when i specifically need to get around BIS. Seems safest that way.
Tick the apn, add blackberry.net. Then make sure that the network is displayed as EDGE for example. Then u know BIS is working. If it edge in lower case its not. Also remember all streaming such as youtube bypasses BIS.