For PBXact? No. You are talking about a business PBX appliance not a hosted voice/VoIP service platform. Yes, you can use A2Billing or any other Asterisk based billing solution I'm sure on the system but FreePBX isn't designed for what you are doing.
FreePBX/PBXact is designed to be a company/business PBX system. If your plan is just to slap some billing solution on your PBXact system and start selling voip services, you are going to be in for plenty of surprises and mostly likely, upset customers.