A gateway device may be configured to receive, from a first user device, a request to establish a Web Real-Time Communications (“WebRTC”) call session; establish, based on the request, a first WebRTC call session with the user device; establish, based on the request: a voice call session between the gateway device and an Internet Protocol Multimedia Subsystem (“IMS”) core, and a second WebRTC call session between the gateway device and a WebRTC portal server; and facilitate, via the established voice call session and the established first and second WebRTC call sessions, multimedia communication connectivity between the first user device and a set of devices that include: a public-switched telephone network (“PSTN”) terminal device, and a second user device.