Remove first 2 digit of a Mobile number

Hi guys,

I have a requirement to remove first 2 digit of a 12 digit phone number and set this trimmed value in a field. How do i do this in a plug in?

Suppose, I have a 12 digit phone number 918989898989 how do i remove the first 2 digit 91 and set the number as 8989898989 in another field of the same entity.

Any Suggestions?

Regards,

Hima