Freelancer must be capable of configuring and setting up A2Billing / FreePBX to start selling DID to customers.
The work that has to be done:
(1) System Settings | Global Settings.
(2) Trunks
(3) Call-plans & Rate Tables
(4) Rates
(5) Asterisk Dial-Plan
(6) Agi-Conf
(7) Create Customer and Test