PSTN Gateways can act as a "Gateway" for different platforms such as TDM\SIP. If your PBX is IP based, then the Gateway can integrate the PBX over SIP and if the PSTN is on TDM, then it can integrate over ISDN or CO Lines. Some gateways have the
capability to provide the dial tone\register end points and some doesn't. It's based on the make of the gateway and your requirement
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