BES is enterprise for a reason, my cousins husband works @ old mutual and they obviously use BES and that literally cuts the BIS server out as their own privately hosted BES server does everything, the vodacom network is simply used as a means of connecting, essentially a VPN link between your phone and the office, even your downloads that you do on your phone from appworld for example go via the companies internet connection and is routed to your device via vodacom. The companies BES server controls absolutely everything including as much as blocking certain handset function including but not limited to making and receiving of phone calls and sms's.
Any email that comes to thier mail server is linked to your phone much in the same way as your computers user account is so with BES you server side mails, contacts, tasks, etc thats stored in your user/email profile are synced with your phone and pc depending. Any contact added to your phone syncs tot he pc and visa versa, same goes for calendar, reminders, memos and tasks.