My area of expertise:
A2Billing Installation, setup & configuration
A2Billing Customization to extend list of features and capabilities
Custom API development
Development and Deployment of Multi Level Security protocols
SMS gateway Integration
SMS Billing system development and integration with A2Billing
Customizing A2Billing and Asterisk to Host and Support Push Notification service (VOIP Push for Apple, Firebase)
SMS Push notification development and integration with Asterisk/A2Billing system