For outbound calls, you can either use a prefix or you can create a pattern that matches valid phone numbers without any prefix. You can also define an outbound route that allows both (dialing with and without prefix).
Let's say you want to dial 11 digits phones numbers in North America with the prefix 9 (e.g., 9 1 613 555 1234) - to do that, you'd add in the "Dial Patterns that will use this Route" area the pattern:
[pre]Prepend Prefix Match Pattern / Caller ID
empty 9 1NXXNXXXXXX
[/pre]To allow calls to be dialed without any prefix (e.g. 1 613 555 1234), the pattern would be:
[pre]Prepend Prefix Match Pattern / Caller ID
empty empty 1NXXNXXXXXX
[/pre]To allow users to enter just 10 digits without any prefix (e.g. 613 555 1234) when the CO needs 11 digits (including the country code 1), the pattern would be:
[pre]Prepend Prefix Match Pattern / Caller ID
1 empty NXXNXXXXXX
[/pre]etc. etc.
You can have a number of different patterns in a single outbound route.