[asterisk-users] Newbie Dialplan: Best Practice in using Context - Do not use Default??

[Lee, John (Sydney)]

In "The future of Telephony", it says "... We should also note for
security's sake you should always make sure that your [incoming] context
never allows outbound dialing.  (If by chance it did, people could dial
into your system and make outbound toll calls that would be charged to
you!)

The book was demonstrating using a PSTN environment and the zapata.conf
was something like:
context=internal
signaling=fxo_ks
channel=>1

context=incoming
signaling=fxs_ks
channel=>2

In PRI environment, does it mean that we have to purposely separate the
say ISDN 20 channels into [internal] and [incoming] as well?  
This would not make sense to me as ISDN uses a one port card to contain
multiple channels while the ports of a say TDM400P refer to each
channel.

If I just define a [default] context for a PRI environment, is this
insecure?

Can someone please enlighten me on this?