Build Asterisk or any opensource PBX central Solution (Turnkey)

We are a small manufacturing organisation with 3 different locations in different states in India. We also have a sales team of around 10~15 employees with another 10~15 employees at the backend. Current telephony solution is OLD legacy Land line phones in conjugation with mobile numbers. The problems that we face: 1. Sales employees leave the organisation and hence their mobile numbers listed on their cards become a sole reason for lost sales. 2. We would want to communicate seamlessly between the locations like in a normal EPABX we dial the extension number and talk to the person. 3. I presume Asterisk has the ability to resolve this issue of ours. 4. We are also looking to have only 1 number to be distributed to all our customers and suppliers so that instead of remembering 20 different numbers our organisation has only one number. The major caveat to this project is that the total recurring cost of telephony should not be more than the current cost we are bearing with the numbers. We are looking for experienced people who can provide a solution for the same and prepare a turnkey solution which is easy to manage at our end.