User Exit for changing NAST entries in SAP R/3

Hi All,

I have a requirement to change the NAST entry before sending the Purchase Order output in SAP R/3 4.7 system.

Actually here we are not using the FAX server and email is used for faxing things. So I would like to change all the FAX entries to Email and put the email id as <fax_number@FAX.COM> and update the NAST entry.

Any Clues on this please let me know.

Thanks in advance.