IP devices<-LAN or private WAN->VoIP Gateway<-->PSTN
There may be a server (such as a SIP server) on the LAN to route the calls from the phones to the VoIP gateway or you may be able to dial the VoIP gateway directly by IP address from the IP devices. You can connect the VoIP gateway's PSTN interfaces (e.g. analog, BRI, T1 or E1) directly to the PSTN.
Related Q&A from Retired Expert - Patrick Ferriter
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