adding a number in front of all my contacts on my cell

[ning.j1]

Hi all,

I really need help here. Here is the scenario: My country recently just added a digit too all cellphone numbers,
eg: old number: 602 1234 | new number: 7602 1234
How would i now change all my contacts to have that extra 7 digit in front?
Please note that this is only for cell phone numbers on my contacts.
Any help to this would be greatly appreciated.
Thanks

J