Obama only uses a blackberry for show when he is in public.
The Whitehouse Communications Office is responsible for maintaining communications with the rest of the government, business and political contacts, and his family. In the event the president needs to take a call, an aide will direct him to a secure tent they setup nearby that is shielded against eavesdropping and electronic surveillance (see http://www.theage.com.au/world/barack-obamas-portable-secrec...). If it is a conversation where they do not expect sensitive topics to be discussed, they might give him a Sectera Edge that is routed over an encrypted satellite link back to the Whitehouse switchboard where the actual call is connected.
