Bitrix24 is a fully featured free virtual PBX that comes with greetings, voicemail, call queuing, ability to record phone calls and save them as mp3 files. It supports unlimited internal employee extensions, call waiting, ability to put callers on hold or transfer them to another agent. You can also forward calls to your mobile number automatically, if they don't get answered by your agents.
If you need virtual PBX to sell or manage existing clients via phone, you'll need a CRM system. Which is included in Bitrix24 free of charge! Importing calling lists or your current customer database takes only a minute. Bitrix24 supports both incoming and outbound calls to/from inside CRM, lets you see client info during call and add notes while you are talking. Your clients are automatically routed to an agent or a manager who's been assigned to them. You can also create leads for missed calls automatically.
Hey, when we said that our free PBX is better than most paid PBX, we weren't kidding. Quotes and invoices? Checks. Absence calendar and employee workload management? Check. File sharing? Videoconferencing? Tasks and projects? Everything is available for free for up to 12 users. And you want to get access to Bitrix24 source code or deploy it on your own server, this option is available too at a reasonable price.