To use 3CX as your phone system, all your phones in the office need to be VoIP enabled.
To use 3CX to call external to a POTS phone, you need a VoIP provider that allows this to happen. What a VoIP provider does is gets your VoIP phone call and routs this to an POTS phone, they do some smarts. What are your remote users? If they are on your company network they can use VoIP phones controlled by 3CX.
I don't think that's the complete answer to the question.
The basic interface to the 3CX is SIP. This means that any incoming call has to be converted to SIP by something before it can be routed by the 3CX. This something may be:
1. A external VOIP provider. (e.g. the SIP packets come in from the outside world over the internet)
2. A SIP device on the network (a SIP handset, a software client on a PC, etc.)
3. A terminal adapter or gateway device (a device that converts analog POTS lines or digital BRI lines) to SIP.
In our case, all of our lines are still POTS (Plain Old Telephone Service) coming into our office. Each handset and each incoming analog line is connected to a analog terminal adapter (ATA) that converts the analog signals to SIP and vice versa. This means that we can migrate piecemeal from our old phone system to VOIP without throwing away what we already have in the office.
In our case, an incoming 888 number comes in over a regular analog POTS line. The POTS line connect to an ATA (analog terminal adapter) (http://en.wikipedia.org/wiki/Analog_telephony_adapter). The output of the ATA is SIP packets that travel over the local network to the 3CX. The 3CX can then route the call back out over the ethernet to:
1. Another ATA that converts it back to analog so it can go out over a different POTS/BRI line (to a remote office). This allows us to connect to those at remote sites that don't have access to any network at all (like my cell phone)
2. Another ATA that converts it back to analog so it can connect to an old analog office handset (for those of us sitting in the office)
3. A hardware-based SIP-phone connected to the network (I'll do this when I get tired of maintaining my old analog handsets).
4. A software-based SIP client running on a computer (I do this at my "remote" sites like my home, since I can route a call that comes over my 888 number throught the 3CX and over the internet to my house where I can answer it on my computer with a simple headset).
In short, you can do anything you want, but you have to have an ATA to convert to/from analog signals (whether coming in from an old handset or from an analog line) or SIP-enabled devices (from your VOIP provider or a SIP-native device or a computer at the far end).
In my case, we don't have any VOIP provider anywhere in our network and we have complete control over where our calls get routed.
Best regards,
Ron
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