But calls will be sent to FreeSWITCH by some device, correct? If it's good old-fashioned SIP then FreeSWITCH will handle it just fine. Yes, there are all softphones (PC machines with old-fashioned headset & mic and smartphones running a sip client). Forgot to add - external calls (via the external VOIP provider) cannot be received via the FreeSWITCH listening port because of the fact that my ISP blocks connections initiated from outside. When a connection request (SYN) is initiated from my side, my ISP allows it, no problem (I believe the applicable term in such scenarios would be "fascist firewall").
What usually happens is that the control channel to my external VOIP provider on port 5060 is initiated by my end and when a call needs to be received from outside, this is communicated through the control channel (which is kept open at all times) and then my client opens both rtp data stream connections to receive the call. Does that make sense?