There are many ways to get the system prefix digits, such as translation pattern or transformation pattern.
For example, you may go to CUCM > Call Routing > Translation Pattern and create a pattern like this:
Pattern: XXXXXXXXXX
Translation Pattern Mask: 91XXXXXXXXXX
This will prefix all 10-digit numbers with 91. Of course, you may fine tune the pattern to make it more specific, such as [2-9]XX[2-9]XXXXXX, etc.
Michael
http://htluo.blogspot.com