Script to add digit "9" to all cell phone numbers

Guys,

at the end of july I´ll have to add the digit 9 in front of all my contacts´ cell phones in the Address Book. But I will not have to add it to other types of phone numbers, such as work and home numbers. I´m looking for a script which would:

1. identify the field "mobile"; and

2. add the "9" digit.

The current cell phone numbers´ structure and the future one will be as follows:

Current number structure:

(021 xx)yyyy-yyyy

Future number structure:

(021 xx)9yyyy-yyyy

Would someone have the slimmest idea of how to do that?

Best regards