For WebRTC connectivity to the PSTN phone, we can adopt one of two approaches: while the first approach is suited to an evolving next generation IMS landscape, the second approach depicts an IN setup. The IN approach is discussed not because all service providers have completely migrated to the IMS landscape, but because the IN service flow and call execution still holds good for many phones. The existing INs have hooks for interconnectivity between cellular phones and old analog phones. This was established using legacy PSTN gateways that took care of the digital-to-analog conversion. The ISUP switch provides the conversion to ISUP, which is responsible for setting up telephone calls in the IN network.

The next generation network that revolves around the IMS setup also provides interoperability with the PSTN system. This will be discussed in detail here. The methodology is adopted from RFC 3398 Integrated Services Digital Network (ISDN) User Part (ISUP) to...