Billing with Spreadsheets

To make customized Invoices to bill clients for incoming calls through our PBX and Credit Notes for telephone agents. We use spreadsheets with background images to make the templates. We make and supply these background images. Before being uploaded to the server, the spreadsheets with background images, including all calculations, must be changed to pdf with open source software, without loosing anything, Than they can be filled out on the Internet by our clients, changed to pdf again and send by email, as invoices or Credit Notes, to their final destination. The client never sees the spreadsheet, he only sees the background image with his entries into the fillable fields ! Requirements: 1) LibreOffice Calc Spreadsheets. 2) Software (open source) to change the spreadsheet files to pdf, without loosing calculations ! 3) Software to connect Asterisk ( CEL - not CRM !) to fillable fields on spreadsheet. N.B.: Our Multi-disk Server and Asterisk PBX are on different networks ! This is not a standard project and not for beginners ! - Very good knowledge of the way Billing software, spreadsheets and Asterisk PBX work is needed. We have a Multi Disk Server with KVM disk management and MySQL DB. -- Good and quick communications are essential ! This is a Linux only project --- No Windows ! ***** Please also read and understand the attachment for step by step instructions -- or ask questions !