How to add SMS system in FrontAccounting ..

Dear Sir,

I want to add free SMS facility to FA, when anyone made a sale or purchase, then a SMS is forwarded to his mobile number, containing Sale & purchase Amount with VAT charged.
For this i have to changed the label of Interface & addition with some codes in purchase & sale modules.So, please provide a overview of files that must contains basic functionality of FA in sales & purchase mdules. What field i use, (i.e. user interface of sales & purchase modules) to store their mobile number.

At All, i want to know the functionality of different files used in sales & purchase modules.

Waiting for your reply .....

Regard,

// Sumit Singh