Magento fax field replace with mobile number

Open file app/locale/en_US/Mage_Page.csv 
Paste the below code at last line
"Fax","Mobile"

For changes in admin panel Go to admin->system->configuration-> customer configuration 
And open the address template tab. Then replace all F (Fax) With M (Mobile) in all tabs
1. Text
2. HTML
3. PDF
4. JavaScript Template
The changes will be work on all order,invoice,shipment history and email all places.